Sir William Jardine.

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^nnsy since thej are nnrepresented in rocks older than the
Upper Llandeilo. The primitive type, however, does not
cease to be represented with the Skiddaw and Quebec groups:
for D. Murchisoni is characteristicallj XJIpper-Llandeilo, ana
2>. aerrcUuhu occurs in the Utica Slate (Caxadoc) of America.
There is, further, one form which would invalidate this gene-
ralization, if it were to be established in the position ori^nally
assigned to it by its author. I allude to the so-called 2>ufymo-
grapsus caduceuMj originallj described bj Mr. Salter fix>m
Canadian specimens (Quart Joum. Geol. Soc. voL ix.), and
afterwards figured by him from the Skiddaw Slates (ibid,
vol. xix. p. 137, figs. 13, a, hy As I have elsewhere stated,
there cannot be any hesitation in rgecting, with Hall, this
species, as far as the Quebec group is concerned ; and an exa-
mination of a very extensive suite of specimens from the
Skiddaw Slates (including Salter's original specimens) has
fully satisfied me that HaU's explanation applies also to Uie
examples from this formation. D. caduceus^ namely, as de-
scribed by Salter, was unquestionably founded upon fragmen-
tary examples of the four-stiped Tetragrapsus firyonoufe^. Hall,
or of the hardly separable Tetragrapatis (Cfrctptolithua) Bigsbyty
Hall. Recently Mr. Baily has stated tiiat Dtdymoarcg^sus
caduceusj Salter, occurs abundantlv in strata of Cara^)c a^
in Wexford ^Quart. Joum. Geol. »dc. vol. xxv. p. 160). l^t
having had tne opportuni^ of seeing the specimens in ques-
tion, 1 do not presume to express any opimon with regard to
them, except tnat, if the name of D. caduceua is to be retained,
it must be made to apply to forms different from those originally
placed under it by mi. Salter. It appears, however, very un-
likely that the ^nus TetragrcmstM^ whidi has hitherto not
been discovered m any Upper Llandeilo deposit, should have
survived into the Caradoc period ; and Mr. Baily's specimens
are therefore likely to be genuine Didymograpsi.

Mr. Carruthers (Geol. Mae. vol. v. p. 129) admits that D.
cadttceusy Salter, has certainly four branches, but still places
it under Didyrriograpsus — ^a position obviously unsuited for it,
whilst he does not recognize its unquestionaole identic with
Tetragrapsus bryonoidesy which he also gives as a Dtdymo-
grapstu^.

* It being now certain that the epecimens ori^nally described by
Salter as D. caducem aie really referanle to that afterwards named by



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Species of Didymograpsus. 349

Dtdymogrcgpsm Murchisonij Beck, sp. PL VII. figs. 7, 7 a, 7 b.

Gri^jtoUtkiu MtirchitonLBeck (SH Syst p.694, pL 26. fig, 4).
Gr^fiokUa Murekiaom, WCoj (PaL Foes, ii p. 6).

Frond consisting of two stipes springing from a mncronate
basC; and including between them an angle of divergence of
from 10® to 15® or 20®. The stipes vaiy in length from a

auarter of an inch up to two inches or more, proceeding from
le radicle outwards and upwards with a sught curve, and
being then continued to their terminations nearly in straight
lines. The width of the stipes varies greatly in different m-
dividuals ; but they are always narrowest at the base, expand
gradually till their full width is attained, and then gradually
contract towards their distal extremities. The ba^ of the
stipe, however, is never so straight as in typical examples of
• D. btfidusy Hall, and the celluliferous margin is not so strongly
convex. Specimens of average size have a breadth near the
base of one twenty-fourth of an inch, and in the fully-developed

Sortion of from one to one and a half line. Gigantic indivi-
uab, however, not unfrequently occur (fig. 7 a) in which
these same measurements are one line and a haJf and one-
quarter of an inch respectively; and even these limits are
occasionally exceeded. The base is obtusely pointed, and is
ftimished with a long triangular mucro or radicle, the length
of which is from one to one and a half line. In the large
specimens, however, the radicle is much less developed pro-
portionally, and is blunt and obtuse. The cellules are on the
opposite side of the frond to the radicle, or occupy the sides of
the angle of divergence, and are from twenty-two to thirty-two
in the space of an mch, having the proximal lip of the cell-aper-
tures prolonged into long acute denticles. In the smaller speci-
mens the cefiules form an angle of about 45® with the axis, are
free for about half their entire length, and have the cell-mouths
somewhat curved and nearly rectangular to the axis. In the
larger specimens, the cellules in the rally-developed portion of
the stipe lose many of these characters, oecoming more nearly
horizontal or rectangular to the axis, whilst they overlap one
another throughout the greater part of their length, and nave
the cell-apertures directed decidedly downwards, owing to the
great prolongation of the proximal margin of each.

Hall Tetragri^^ {QraptoUikiu) (ryonou^M. Salter's specific name ahould
haye the priority, as bearing the date 18od| whereas Hall's name was
given in 1857. The spedes, therefore, should be called Tetraarcmma
cadiioeiu9j Salt, sp. There appears, however, to be no doubt that the form
is really identical with the JWomm terra of Bronffniart, published in
1828. In strict justice, therefore, the species should -be called Tetra-
grapam gerra, Brongn., sp.



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350 Dr. H. A. Nicholson on the British

This well-marked species has long been known to all stu-
dents of Silurian geologjr, but has never been fully described.
It is characteristicallj Upper-LlandeilO; and I am not aware
that it occurs in any other formation. One of the most re-
markable points about this form is the extraordinary dispro-
portion in size between different individuals. Numerous inter-
mediate examples, however, occur, connecting the smallest
and largest individuals ; so that there can be no doubt as to
their specific identity.

Loc. Upper Llandeilo rocks of various parts of Wales,
Abereiddy Bay in Pembrokeshire being one of the most noted
localities. Llandeilo rocks of County Meath, in Ireland
(BaUy).

Didymograpsus divaricatusy Hall, sp. Pi. VII. figs. 4 & 4 a.

OraptoUOma dwariccshu, Hall (Pal New York, vol. iiL SappL p. 513).
D%cranogfxq^9U9 divarictxtus, Hall (Grapt Quebec Group, p. 57).
Didymograpmu elegam, Carruthers (in part); Geol. Mag. toL t. pL5. ^. 8a.
Didymograptus Moffatentia, Carruthers, Ann. & Mag. Nat. Hist. Jan. 1859.

Frond consisting of two long and narrow stipes springing
from a mucronate base, attaining each a length of from two to
three inches or more, and including between them an " angle
of divergence" of from 90° to 130°. The base (fig. 8d) is
convex and rounded, and is formed by a long triangular me-
dian radicle, flanked by two shorter lateral spines, the whole
three occupying a non-celluliferous space of over one line in
breadth. The radicle is in its normal position on the inferior
aspect of the frond, and the cellules are on the same side of the
frond as the radicle. In this species^ therefore, as in i>. sex-
tanSy the true angle of divergence is bounded by the non-
celluliferous margins of the stipes. The " radicular angle," or
that on the same side of the frond as the radicle, is in this case
contained between the celluliferous margins of the stipes, and
varies from 270° to 230°. Each stipe is about one-fortieth of
an inch in breadth at its commencement, and gradually widens
out till a width of half a line may be attained. The cellules
are from twenty to twenty-six in the space of an inch, their
outer mardns curved, convex, and nearly parallel to the axis,
the denticles obtuse and rounded, and the cell-apertures form-
ing oblique indentations or poucnes which extend about half-
way across the stipe, and are.rounded-off internally. Accord-
ing to Hall, " the surface is marked by a row of small nodes
placed obliquely to the direction of the axis, and situated just
Delow and a little on one side of the bottom of the serrature."

This beautiful species (originally described by Hall from
the Hudson-River group of America) is distinguished from all



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Species of Didymogra,^B\X3. 351

others by the possession of a median radicle and two lateral
spines, placed on the same side of the frond as the cellules.
JD.Jlaccidus^ Hall, has three smaller spines placed in a similar
manner on the same side of the frond as the cellules (fig. 8) ;

Fig. a




a, base of D.JUtcctdiu, Hall ; b, base of D. anceps^ Nicb.^ sbowin^ the in-
ternal radide ; c, base of another example of 2>. anctps, in which tiiere
is no radicle ; dy base of 2). divaricatus, Hall; showing the radicle with
its two lateral spines. All enlarged.

but the central spine of these is not the radicle^ as is shown
by the occurrence of the true radicle on the omxmte side of
the frond — ^this completely altering the whole relations of the
parts. These anti-radicular ornamental spines of D.Jlaccidus
nave^ however, been confounded by Mr. Carruthers with the
genume radicle with its flanking spines in I), divaricatus.
As regards the form of the cellules D. divaricatus cannot be
distinguished from D. sextans^ Hall, and D. anc^s, Nich.
The former, however, of these is readily distin^snea by its
general form, and the latter, as I shall immediately explain,
is separated by the fundamental structure of the frond.

Dxdymograpsus Moffatensisy Carr., and one of the specimens
included under D, eleganSj Carr., are clearly identical with one
another ; and both (unless figured upside down) appear to be
referable to D, divaricatus j Hall, which bears the date of 1855,
and has therefore the priority*.

Loc. Rare in the anthracitic shales of Glenkiln Burn, in
Dumfriesshire (Upper Llandeilo).

Didymograpsus ancepsy Nich. PI. VII. fig. 5, 5 a, 5 b.

(Geol. Mag. vol. iv. p. 110, pi. 7. figs. 18-20.)

Frond consisting of two stipes, diverging from an initial
point which may or may not be marked by the presence of a

« It is qmte possible that Didymograpsus (Cladoarcpsus^ Ibrchammeri,
Geinitz, is really identical with D, divaricatus, Hall, in which case Qei-
nitz's name would have to be retained, as it was published in 1852.
Accepting, however, the aocnracy of the figure ffiven by Geiniti n>ie
Gnwt. pL 5. figs. 28, 29), the base appears to be destitute of the raoicle
and lateral spines so characteristic oil), divaricatus. The other figures of
Geinitz (ibia, pi. 5. fiffs. dO, 81) are certainly referable to a difibrent form,
probably to D.flaccidus, Hall.



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352 Dr. H. A. Nicholson an the British

radicle. In some cases the initial point is recognized simplj
bj the fact that it is the point of flexion of the frond, and from
it the cellules point in opposite directions. In other specimens
the initial point is markea W the presence of a slender radicle,
the length of which varies from a mere node up to nearlj one
line, m all specimens which exhibit anj traces of a radicle,
without exception, this is on the inforior^ whilst the cellules
are on the superior aspect of the frond, so that the two are on
opposite sides (fig. 8 b). The result of this is, that tibe ^^ angle
of divergence," properly speaking (namely, the angle formed
by the stipes on the opposite side of the frond to the radicle),
is in this case to be measured between the cellulif^rous mar-
gins of the stipes ; and it varies from 340^ to 355^ The '^ radi-
cular angle," on the other hand, is included between the non-
celluliferous margins of the stipes ; and it varies from 6*^ to 20^.
The margin of the frond opposite to the radicle is never orna-
mented by spines, and is simply formed by the coalescence of
the bases of the first two cellules. This structure is of interest,
as agreeing with D. sextans, Hall (at any rate, in its ordinary
form), and apparently foreshadowing what we find in Dicrcmo-
grapsus. The stipes are very little narrower at their origin
than elsewhere ; and they retain a pretty uniform width through-
out, varying in different individuals from one twenty-fourth
of an inch up to two thirds of a line. The cellules are not
distinguishable in shape from those of D. divaricatus, HaU,
and D. sextans. Hall. They are from twenty-five to thirty in
the space of an inch, their outer margins convex and nearly
parallel to the axis, their apices rounded off, and the cell-
apertures forming oblique pouch-like indentations, which ex-
tend halfway across the stipe. In some specimens, the first
few cellules on either side of the initial point are provided
each with a short blunt spine proceeding from the centre of
their outer margins. In some examples there are minute
pustules or circular depressions in the centre of each denticle
where it joins the body of the stipe ; but this phenomenon is
not constant in its occurrence. As I have already said, in the
shape of the cellules D. anceps is not distinguishable from D.
divaricatusj Hall ( =i?. Moffutensis, Carr. ?). In all other re-
spects, however, they are totally distinct ; and they could only
be confounded, as mey have been (Camithers, Greol. Mag.
vol. V. p. 129), by turning 2>. anceps upside down. In the
first place, in I), anceps the radicle and cellules are on opposite
sides of the frond, whilst in D. divaricatus they are on the
same side. In audition to this very obvious and, indeed,
fundamental distinction, the following points of difference may
be mentioned : — In Z>. anc^s the " angle of divergence," as



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Species o/'DIdjinograpsus. 353

measured between the stipes on the side opposite to the radicle^
is from 340° to 355°, the radicle is not nimished with lateral
spines^ and the width of the stipes is extremely uniform in
any ffiven individual. In D. divartccUtis. on the other hand,
the "angle of divergence," measured in the same way, is from
90° to 130°, the radicle is invariably flanked by two lateral
spines, and the stipes are considerably narrower at their com-
mencement than towards their distal extremities. These
points dof difference should be sufficient to prevent in fiiture
any confosion between two species which in reality belong to
two different sections of the Didymwrapsi.

Loc. Upper Llandeilo, Dobbs's Linn, near Moffat.

Didymograpsus jUiccidus^ Hall, sp. PI. Vll. figs. 6, 6 a, 6 &, 6 c.

Orapiotithus Jiaceidui, Hall (Qrapt Quebec Group, Suppl. p. 143, pi. 2.
'* . 17-19).

raptmjlaccidm (Nicholson, Qeol. Mag. voL iv. p. 110).
^rapms degamy Carruthers (in part), Geol. Mag. vol. v. pi. 5.
figs. 8 6, 8 c.

" Frond consisting of two slender, linear, fleinious stipes,
which are widely divergent from a small, short, obtuse radicle
(Hall). The stipes are about one fiftieth of an inch in breadth
at their commencement, but widen out till a width of one
twenty-fifth of an inch may be attained, and they not unfre-
(^uently reach a length of several inches without snowing any
signs of a termination. The proper " angle of divergence " of
the stipes, as measured on the opposite side of the frond to the
radicle, is from 280° to 320°, whilst the " radicular angle " is
from 40° to 80°. The radicle varies in length from one
twenty-fourth of an inch up to one tenth, bemg sometimes
long and pointed, at other times short and obtuse, whilst it is
invariabh" situated on the inferior or concave margin of the
frond. The margin of the frond immediately opposite to the
radicle is adorned by three short and delicate processes or
spines— one directly opposed to the radicle, and one springing
from the first cellule on each side (fig. 8 a). These spines are
simply ornamental appendages, so to speak, and have nothing
whatever to do with the true radicUy itom. which they must be
carefully distinguished. The cellules are on the opposite side
of the frond to the radicle, from twenty-five to tnirty in the
space of an inch, averaging twenty-eight, narrow, their outer
margins straight or veiy slightly curved, inclined, to the axis
at a very low angle (alJout 20**), their apices usually gently
rounded, and the cell-apertures running partially across the
body of the stipe..

As to the complete identity of this beautiful species with



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354 Dr. H. A. Nicholson on the British

the Chapiolithu$ facddua described by Hall from the Utica
Slate, there can be no doubt ; and in this opinion I am fnlly
borne out by Prof. Harkness, who has exammed some of my
specimens. Onr British specimens have been placed by Mr.
Carrothers under his2>. e^atULwhich seems tobefounded partly
upon D. divaricatusy Hall, ana partiy upon D.flacddus. The
specimens figured by Mr. Carruthers as D. degans^ and really
belonging to D.flaccidusj are figured upside down (GreoLMag.
vol. V. pL 5. figs. 8 ft, 8 c).

Our British examples, however, agree with D.flacctdus, as
described and figured by Hall, in tne general shape of the
fix>nd, in the position of the raoicle, in tne shape of the cel-
lules and in their number to the inch, and, in fact, in evei^
essential respect, except in the fact that the American speci-
mens appear to want the small spines which are found oppo-
site to the radicle in our form. These, however, are not con-
stantly preserved, even in the British specimens ; and even if
constantly wanting in the American examples, their absence
would not be enough of itself to constitute a specific distinction.
From D, divaricatusy Hall, the present species is distinguished
by the fact that the cellules are on the opposite side of the
frond to the radicle, the reverse being the case in the former ;
whilst the characters of the cellules in the two show several
decided points of difference. From D. ancepsj Nich., in which
the cellules and the radicle hold the same relative position as
in D.JlacciduSy the latter is distinguished by the much greater
length and tenuity of the stipes, as well as by the difierent
characters of the cellules.

I have only to add that, in connexion with the fully grown
fronds of this species, there often occur numerous young forms
in different stages of development, commencing with those
which exhibit only one or two cellules on each side of a cen-
tral radicle (PL VII. fig. 6 c). Even in these small forms,
however, the three minute spines opposite to the radicle can
be recognized.

Loc. XJpper Llandeilo rocks of Dobbs's Linn, and Hart Fell,
near Mofl&t.

Didymograpsus sextans^ H^ll, sp. Fig. 9.

Graptdithus sextans, Hall (PaL New York, voL i. p. 273^ ^ 74 figs. 3 a-«).

Dipograpms (P) sextans, M'Coy (Pal. Foes, part 2, p. 9).

OrtyaioUthus sextans, Salter (Quart. Joum. Geol. Soc. vol v. p. 17, pi. 1.

fig.io).

Dicranoffrapsus sextans, Hall (Qrapt Quebec Group, p. 67).
Didymograpsus sextans, Baily (Characteristic British Fossils, pi. 9. figs.
Qti-d),

Frond consisting of two small stipes, generally from four to



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Species of Didymograpsus. 355

five lines each in length, with an average breadth of about
half a line, diverging from a mucronate base at an angle of
about 60^. The base is rounded, and is seen, in the few
specimens which are well preserved, to be provided with two

Fig. 9.





b



Didmnograpsus sextans: a, a specimen slightly enlarged and with the
cellules partially restored ; h, base of the same^ enlarged.



lateral spines, and sometimes with a central minute spine or
radicle, though this latter can only rarely be detected. The
radicle is, as usual, on the inferior aspect of the frond, and the
cellules are situated on the same side — ^a peculiarity found in
no other Didymograpsxis except D, divaricatusy Hall. The
" an^le of divergence " is therefore included between the non-
cellmiferous margins of the stipes ; and it is almost always
about 60°. The " radicular angle " is bounded by the cellmi-
ferous margins of the stipes, and is, of course, about 300°.
The cellules are from thirty to thirty-five in the space of an
inch, and the first two are coalescent by their bases, as in Z>.
anceps. In all essential respects the cellules are identical with
those of D. divartcatus and D. anceps. The outer cell-walls,
namely, are curved and subparallelwith the axis ; the denti-
cles are obtusely rounded off; and the cell-apertures form
oblique indentations extending about halfw^ay across the stipe.
These, at any rate, are the characters of the cellules in our
Britisn specimens, in those few examples in which they admit
of examination, as they rarely do. In Hall's orimial descrip-
tion the cellules are said to terminate in " slen&r mucronate
points ;" but some error must undoubtedly have been made
upon this head. This is rendered certain by the fact that Hall
has subsequently placed D. sextans in the genus Dicranoarapsus
along with Z>. divaricatus. expressly upon the ground of the
similarity in the shape ot the cellules, whilst he has figured
the latter with, cellules such as I have described above.

The propriety of placing D. sextans in the genus DtcranO'-
ffrapsus as this genus is understood by British palsdontologists,
may still be looked upon as an open question. In none of the
many specimens which have passed through my hands have I
observed any thing more than the coalescence of the first two
cellules by their bases. This, though perhaps an approxima-



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356 On the British Species of DidTmograpsus.

tion to DicranoffrcgMuSf occurs also in D. ancepsy and is not
sufficient to require the removal of the species from Didymo-
aramue. Beoentlj. however, Mr. John Hopkinson has been
Kind enough to sena me drawings of some specimens which
appear to belong, beyond a question, to D. sextans j but in
"v^ch this amalfflimation has gone further. In these, namely,
whilst the bulk of the frond has all the characters of 2>. sex-
tanSf there is an exceedingly short basal portion formed b^ a
coalescence of the first two or three cellules on each side.
Whether this form is identical with OraptoUthus JiircatuSj
Hall (Pal. New York, vol. i. pL 74. fisp. 4a-A), or whe&er it
should be looked upon as a transition between Z>. sextans and
Dicranograpstis proper, I am unable to sav. D. sextans^ in
its typical form, as above described, is easilv recognizable by
the shortness ot the stipes, the constancy of the angle of di-
vergence, the presence of the radicle and the cellules on the
same side of the frond, and the characters of the cellules.

Z/Oc Abundant, but badly preserved, in the anthracitic
shales of Olenkiln Bum in Dumfriesshire, and Cairn Byan in
Ayrshire; also in several localities in Ireland (Baily).



EXPLANATION OF PLATE VH.

Fig, 1. Didymograpsus patidmj Hall, nat size. From the Skiddaw Slates
of Outemde, near Keswick.

1 a. Portion oi D. patuhUf enlarffed, to show the oelluleSy after Hall.
Fig, 2. Didymograpmu extensus, HaU, nat sixe. From the Skiddaw

Slates of Outerside, near Keswick.

2 a. Fragment of D, extenna, enlarged, to show the cellulee, after

Fig. 3. Didymograngus ierratuhu, Hall, nat. sixe. FVom the Skiddaw
Slates of Outersid^ near Keswick.
8 a. Base of D, serratukuj enlarged, after HalL

3 b. Base of D. urratuktSy from another specimen, from the Skiddaw

Slates of Outerside. Enlanrod.

3 c, D, Hrrattdus (P), from the Skiddaw Slates of Outerside, natural

size. The angle of divergence is much greater in this than in
ordinary specimens.
Zd. D, 9err(Uuiu8^)f from the Skiddaw Slates of Thomship Beck,
near Shap. The angle of divergence in this specimen is much
less than in ordinary specimens. Natural size.
Fig, 4. Didymo^apgus divarieatusj Hall, slightly restored from a Dum-
friesshire specimen.

4 a. Base of a specimen of 2). divarieatua, from the'^Upper Llandeilo

rocks of Dumfriesshire. Enlarged.
Fig. 5. Didymograp9u$ anoepsj Nich., sH^tly enlarged. Upper Lluideilo
ro^ of Dobh6*s Lmn, near Mofiat

5 a. Base of another specimen of the same, enlarged. In this speci-

men there is no radicle.

6 h. Base of another specimen of the same, in which a radicle is pre-

sent; enlarged.



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Mr. A. G. Butler on Butterfliea from the South Seas. 357

I^.Q. Didymograp9us JtacdduB. Hall, natural sise. From the Upper

Llandeilo rocks of DobWs lAxm, near Mofiat
6 a. Base of another specimen of the same, enlarged, showing the

three small spines opposite to the radicle.
6 6. Fragment of the same, enlarged, to show the cellules.

6 c. Germs of D.flaccidu8, nat size.

Fig, 7. Small specimen of Didymoyrapmu Murchisoni, Beck, nat size.
From the Upper Llandeilo rocks of Abereiddy Bay, in Pem-
brokeshire.

7 a. Large specimen of 2>. MwMsom, from the same locality, nat.



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