British Museum (Natural History). Dept. of Zoology.

Guide to the specimens of the horse family (Equidæ) exhibited in the Department of Zoology, British Museum (Natural History) online

. (page 1 of 5)
Online LibraryBritish Museum (Natural History). Dept. of ZoologyGuide to the specimens of the horse family (Equidæ) exhibited in the Department of Zoology, British Museum (Natural History) → online text (page 1 of 5)
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Owing to the great interest attaching to the Horse and its.
relatives (alike from the point of view of the naturalist, the
breeder, and the sportsman) it has been decided to issue a
special Guide- Book to the specimens of this group exhibited
in the public Galleries of the Zoological Department of the

It is hoped that the following pages will not only tend to
stimulate this interest, but that they will also lead to the
presentation of ''^ record ''^ specimens for preservation in the
National Collection. In the case of skeletons of celebrated
thoroughbreds, Arabs, and other breeds, their true interest can
only be fully appreciated when they are brought together in
a large series. It is only in a great public museum that the
proper care and preservation of sucli specimens can be assured.

The present Guide-Book is the work of Mr. II. Lydekker.



Bkitish Museum (Natural History).
October, 1907.





-. Although frequently employed in zoology in a wide

I. u .. sense^ to indicate all the members of the family
Equid(Sj both living and extinct_, the term " Horse "
properly denotes only the well-known domesticated animal Equus
caballus and its half-wild or wild representatives. Since, more-
over, the Latin name was given by the Swedish naturalist Linnaeus,
it seems necessary to regard the domesticated Horses of Scandinavia
as the typical representatives of the species.

In these pages the term Horse is niainly used in the more
restricted sense.

Different views are entertained with regard to the limitations of
the family Equidce, some naturalists including in it all the extinct
animals belonging to the same line of descent, or "phylum/^
while others restrict it to those more or less nearly related to the
living species.

_. 11 In the latter sense the Equidce are characterized by

the tall prismatic crowns and complex structure of
^* their cheek-teeth, in which all the hollows and valleys
formed by the infoldings of enamel are filled by cement, so as to
form a grinding surface of a perfect type. Another feature is the
presence of an infolding of the enamel in the summits of the
incisors, thus producing what is called the '' mark.^^ In the skuil



the enclosure of the socket of the eye by a complete bony ring is a
feature distinctive of the group. In all existing members of the
family, constituting the genus Equiis, there is only one toe on
each foot, although rudiments of lateral digits are represented
by the '' splint-bones '^ on each side of the upper end of the

Dentition '^^^^ dentition of the Horse is illustrated on one side of
of the the table-case placed near the middle of the North Hall;
Horse. the object of the specimens being to show the alterations
which take place with age. In all its features the dentition
displays special adaptation for the masticating of vegetable food,
such as the herbage of the open plains upon which the species
dwells in a state of nature. The front teeth or incisors are, for
instance, used for cropping off the blades and stems of grass,
while the cheek-teeth (molars and premolars) serve for crushing
and breaking them into fragments. The tusks, or canines, so
greatly developed in carnivorous animals, are comparatively small
even in the males, and rudimentary in the females.

The complete number of teeth in the addt Horse is that
characteristic of Ungulate or Hoofed Animals of the early Tertiary
period, viz.,thv^e incisors (i), one canine {c), four premolars (p),
and three molars (m) on each side above and below, or forty- four
in all. The first premolar {p. 1) is, however, very small, and
often wanting, especially in the lower-jaw; but instances of its
presence are shown m several specimens in the case ; these being
of interest, as remnants, on the point of disa})pearance, of a tooth
well developed in the Horse-like Animals of ancient times.

The incisors, as mentioned above, have an infolding of the
surface, constituting a deep pit (the '^mark^^), a feature now
confined to the Eqiiidce. In consequence of this pit extend-
ing only a certain depth into the crown, it becomes obliterated
as the tooth wears away, so that its presence is a guide to the
age of the animal. The six principal cheek-teeth are in close
contact by broad surfaces fitting tightly against each other, so
that they collectively form one solid mass, presenting a grinding-
surface composed of substances of various degrees of hardness
(enamel, dentine, and cement), interwoven into an intricate
pattern so as to make most efficient natural millstones. The

Fig. 1.






The Ancestoks of the Horse and its relatives compared in size and


a. Hyracotherium or Protorohippus, of the Lower Eocene ; h. Plagiolophus,
or Orohippus, of the Middle Eocene ; c. MeHohipjms, of the Oligocene ;
d. Merychippus, of the Miocene ; e. PUohippus, of the l^liocene ; f. The Modern
Horse, Equus caballus, domesticated breed. (LiiW, Amer. J. Sci. vol. xxiii, p. 167.)

[To /ace page's.



grinding-face of the tooth always keeps at the same level, the
gradual wear of the superficial parts being compensated by the
pushing outwards of the whole tooth in its socket until, as may
be seen in the older specimens, nothing but the root is left.

The permanent teeth are preceded by a temporary or deciduous
set of " milk-teeth " {d.i., d.m., &c.) ; in which there are as many
incisors as in the permanent set, although there are only three
cheek-teeth on each side above and below ; these milk-molars
being replaced by the last three permanent premolars. The
eruption, or cutting through the gums, of the deciduous teeth
commences at about the time of birth, and is completed before
the end of the first year, when the young animal has its full set ;
the upper teeth, as a rule, appearing somewhat earlier than those
of the lower jaw. The first teeth to appear are the central incisors
and the molars ; between the first and second months the second
incisor appears, and finally (at about nine months) the third
(corner) incisor, which completes the milk-dentition. Of the
permanent teeth, the first molar appears about the end of the first
year, followed by the second molar before the end of the second
year ; these teeth being thus in place before any of the milk-teeth
have been shed. At about two and a half years the second and
third premolars replace their predecessors ; and between two-and-
a-half and three years the first permanent incisor appears. At
three-and-a-half to four years the fourth premolar, the third true
molar, and the second incisor have appeared ; while at four-and-a-
half to five years the third (corner) incisor and the canine have
cut the gum, thus completing the permanent dentition. Up to
this period the age of the horse is clearly shown by the condition
of its teeth, and for some years longer indications can be obtained
from the wear of the incisors, though this depends to a considerable
extent upon the hardness of the food and other accidental

In the specimens exhibited the side view of the teeth of the
right side, and the grinding-surface of the teeth of the left side
are shown.

The series of skulls exhibited comprises specimens ranging in
age from the unborn colt to a horse of 36 years.



_, , , , Facing: the visitor as he enters the middle of the

Skeleton of .\ „ , . . w i i f

north hall are shown m a single case the skeletons
Man & Horse. ^^ ^ -^^^ ^^^ ^^ ^ ^^^.^^ (N.H. 1), arranged for

comparison with each other, and also to show the position of the
bones of both in relation to the external surface. In the case of
the Horse, the skin of the animal from which the skeleton was
prepared was mounted, and, when dry, divided in the middle line^
and one half, lined with velvet, placed behind the skeleton. In
the Man the external surface is shown by a papier-mache model,
similarly lined and placed in a corresponding position. The
principal bones of both skeletons have their names attached,
so that study of this group, besides affording a lesson in com-
parative anatomy, may be of practical utility to artists. The
meanings of the terms pastern, fetlock^ etc. are also explained in
this specimen.

Evolution Specimens illustrative of the evolution of the Horse are

of the displayed on the north side of the table-case near the

Horse. middle of the north hail ; that is to say in the same

case which contains, on the south side, the series illustrating the

dentition of the Horse.

The evolution of the Horse (and its allies) is better known
than that of any other group of Mammals. In passing from
the Horse to its earlier ancestors, a gradual decrease in bodily
size (fig. 1), accompanied by a shortening of the lower segments
of the limb, especially of the bones of the foot, is very noticeable ;
at the same time there is an increase in the number of the toes,
while the height of the crowns of the cheek-teeth is lowered, and
their structure becomes simpler.

In the Horse, in common with the other members of the genus
Equus, the skull (fig. 13) has the socket of the eye completely
surrounded by bone, there is no distinct depression immediately in
front of the same, the canine and incisor teeth are separated by a
lon^ gap from those of the cheek-series, and the crowns of the
latter are very tall and continue to grow till late in life, while their
grinding-surfaces are much complicated, owing to the fiUing-up of
all the cavities with the substance known as cement. Each limb
terminates in a single hoof, upon which alone the animal walks ;

Fig. 2.

Skeleton of Foj{e-feet oe extinct Fojie-iunner.s of the Horse :

A. Hyracothcrium (No. N. 11. (w) ; ]*,. MesoJdppm (No. N. II. ~u) ; V. Merychippus,

or Protohippus (No. N. li. 57) : D. Kippavion (No. N. H. 44).

[ To face page 5.


the lateral toes being represented only by the so-called ^^ splint-
bones '' (tig. 7).

Remains of Horses indistinguishable from some of the various
forms of the existing species occur in the superficial deposits of
Europe and Asia, in company with those of the Mammoth. At a
somewhat earlier epoch (Pliocene) occur Horses, such as E. stenonis
of Europe and E. sivalensis of India, in which the head is
relatively larger, the feet are somewhat smaller, the splint-bones
more developed, and the skull shows traces of a depression in
front of the eye. The American Pliohippus is smaller, with
shorter cheek-teeth. Still earlier (Miocene) is found in America
a Horse known as Merychippus or Protohippus in which the
splint-bones are fully developed and terminate inferiorly in small,
although perfect, toes. In the early Pliocene Hipparion, or
three-toed Horse, the lateral toes are still larger, while the crowns
of the cheek-teeth are lower, and the skull is shorter and shows a
large depression in front of the eye. In this animal the crowns of
the cheek-teeth are still tall and have their hollows filled with
cement (fig. 6, E), and there must consequently be some unknown
forms connecting it with the Miocene Anchit her turn, in which the
crowns of these teeth are quite short, and have their hollows free
from cement. Hipparion is generally regarded as ofi" the direct
ancestral line.

This type is common to Europe, Asia, and North America ; but
Mr. J. W. Gidley, in the Bulletin of the American Museum, has
come to the conclusion that the New World Hipparions are
generically distinct, and proposes that they should be known as
Neohipparion. They differ from the Old World forms by certain
details of tooth-structure, as well as by their more slender limbs,
in which it seems that the lateral toes are relatively smaller.
Finally, they are of Miocene, instead of Pliocene, age.

Nearly allied to Anc hit her iu7n is the Oligocene ^enns Mesa hippuSj
the species of which are smaller than the typical representative of
the former. In these animals the socket of the eye is open behind,
the gap between the canine and cheek-teeth is comparatively
short, the lateral toes are functional, and there is even a suggestion
of a fourth toe in the fore-foot (fig. 2, B). This digit is fully
developed in the fore-foot (fig. 2, A) of Hyracotherium^ a Lower


Eocene Mammal not larger than a Fox, in which the lateral digits of
both feet are relatively as large as in the Tapir, while all the bones
of the feet are proportionately shorter than in the Horse, and all
three joints of each toe probably touched the ground. Species
intermediate between Mesohippus and Hijracotherium have been
named Flaffiolophus and Orohippus. Farther it is not at present
possible to carry the ancestry of the Horse, but there is little doubt
that Hyracotherium is descended from a still earlier five-toed
Mammal with a simpler type of cheek-teeth, and much shorter
foot-bones. This hypothetical animal doubtless walked on the whole
sole of its foot (plantigrade progression) instead of on the tips of
the toes, and was probably nearly related to the creature known
as Phenacodus, a cast of the skeleton of which is exhibited in
the Gallery of Fossil Mammals. For further details concerning
the extinct allies of the Horse see 'A Guide to the Fossil Mammals
and Birds in the Department of Geology and Palaeontology \
rt ., - . The superficial (Pleistocene) deposits of South

South American , . ^ ■ ^^ ^ ^ .^

_ ,. , „ America — more especially those oi the province or

Extmct Horses. r> a i, n i ^- ^

Duenos Aires — have yielded remains oi two very
remarkable equine animals, Hippidium neogaum and Onohippidium
munizi. Of the former the model* of a nearly entire skeleton
(N.H. 3, fig. 3) is exhibited, while the latter is represented by a
cast of the skull (N.H. 17). In both genera the cheek-teeth
(as mentioned later) have shorter crowns and differ in several
details of structure from those of modern Horses. As mounted,
the skeleton stands 4 ft. 1 in., or 12^ hands, at the withers, while
the skull measures 23^ in. in total length. In an average
European horse-skeleton, standing 4 ft. 9J in., or 14 hands Ij in.
at the shoulder, the skull-length is about 23} in., or practically
the same as in the much smaller Hippidium. Although these
measurements suffice to show how disproportionately large is the
skull of the Hippidium, they by no means indicate the chief
peculiarities of that animal. Com.parison of the skull of the
former with that of an ordinary Horse shows a most remarkable
difference in the structure of the nasal region of the two species.
In me ordinary Horse the nasal bones are separated from the
maxillae, or upper jaw-bones, of either side by a slit of only some

* The original of this model has been made tlie type of a second species,
but on very slight grounds.

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Fig. 4.

Skull of Onohippidmm munizi. (From the model in the Museum,
No. N. H. 17). pf. preorbital fossR; or depression. About \ nat. size.

Fig. T).

Skull of a small S. American Derr, Pudua jfudu (No. N. H. 17), to show the
preorbital fossa, or depression (pf.), which contains a gland. ^ nat. size.

[ To face page'Jl.


three or four inches in length. In Hippidium (as in Onohip-
pldmm, lig. 4), on the other hand^ these slits are about 10^ in.
long, while the nasal bones themselves are proportionately long
and slender. This clearly indicates that these extinct American
Horses had extremely elongated noses, not improbably forming a
kind of short trunk comparable to that of the Saiga Antelope.

In that animal, as well as in its relative the Chiru Antelope of
Tibet, the increased size of the nasal chamber has been brought
about by a shortening instead of an elongation of the nasal bones,
but it is probable that in these two Antelopes and in the Hippid-
ium the purpose of the modification is the same. It has been
generally supposed that in the case of the Chiru the large size of
the nasal chamber is an adaptation to the respiratory needs of an
animal living at a very high elevation. In the case of the Saiga
such an explanation would, however, obviously not hold good;
and the real explanation in all three cases may perhaps be found
in a special adaptation to a desert life, the long nose serving as a
filter to prevent particles of sand reaching the organ of smell.

As regards the rest of its skeleton, Hippidium is remarkable for
its short and stout limbs ; this being chiefly due to the excessive
shortness of the cannon-bones, which are also unusually wide,
with very stout splint-bones. Each limb terminates in a single
toe. These short limbs, coupled with the huge unwieldy head,
indicate that Hippidium had less speed than ordinary ponies.
There are only five lumbar, or ribless trunk, vertebras, as in the
Arabian Horse.

Two other points of interest in connection with these peculiar
equine animals deserve brief reference. From the conformation
of the bones of the nasal region it seems certain that neither
Hippidium or Onohippidium can be derivatives from the genus
Equus, while it is still more evident that Equus cannot be
descended from Hippidium. Consequently, the reduction of the
digits from three in the ancestral Horses to a single one on each
foot has taken place independently in the two genera. The second
point is that if the wild Horses alleged to have been seen by Cabot
in Argentina in the year 1530 really were, as some suppose,
indigenous, they must have been either Hippidium or Onohippidium^
and not Horses of the Old World type. With the evidence
afforded by the skins of the Patagonian Ground-Sloth as to the


comparatively late date to which that species survived, there is no
vahd reason why Hijypidium and Onohippidium should not have
survived till Cabot^s time, especially as their hoof's have been
found in comparativel}/ fresh condition alongside the remains of
the Ground -Sloth.

While the skull of Hippidium shows no marked depression in
front of the eye-socket, that of Onohippidium (fig. 4) has an
enormous pit in this position, with a smaller and partially
detached one in front.

_ -, - In the same case with the skull of Onohippidium

_ ,. . „ are exhibited a few specimens (N.H. 34) illus-

Extinct Horses. . . .i . . \ A-rc ^u

tratmg the structural diiierences m the upper
cheek-teeth of some of the later members of the Equidce, and also
the marked difference between an unworn and a worn molar of
Equus cahallus. The specimen marked A (fig. 6, A) is an unworn
molar tooth of the latter species, with the infoldings of the
crown not yet filled with the cement, which is developed later.
D (fig. 6, D) shows the condition of a similar tooth which has
been some time in use. The summits of the columns coloured
red in A have been worn away in D so as to expose the dentine
or ivory (red) forming the interior of the tooth ; the infoldings
on the crown, of which the central ones are converted into islets,
being filled with cement (blue) . The enamel, forming the proper
external surface of the tooth, is left of the natural colour.
Specimen C (fig. 6, C) is a half-worn tooth of the above-mentioned
extinct South American Hippidium, in which the two disks
(anterior and posterior pillars) on the lower border coloured red
are more alike than in Equus ; the whole crown of the tooth being
also shorter. Specimens B and E are respectively slightly worn and
half-worn teeth (fig. 6, B & E) of the European Three-toed Hipparion.
In these the anterior pillar (a) is isolated from the rest of the
crown, thus indicating that the genus is off the direct line of
ancestry of the modern representatives of the Horse family.
n II ■!. Although it is unnecessary to discuss the «:eneral

Callosities or * , ... ° .

, I " structure of the EquidcBj it is important to mention

that all members of the Horse tribe have a
bare patch of hardened skin on the inner side of the fore-leg,
situated some distance above the carpus, or "knee." In the
Horse a similar but smaller callosity, or " chestnut " generally.

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Online LibraryBritish Museum (Natural History). Dept. of ZoologyGuide to the specimens of the horse family (Equidæ) exhibited in the Department of Zoology, British Museum (Natural History) → online text (page 1 of 5)