- 
ON BRITISH FOSSIL MAMMALIA. 927 
are in close juxtaposition. The third and fourth premolars present a sudden 
increase of size and of complexity of the grinding surface, with a correspond- 
ing change of form. The plane or transverse section of the crown 1s sub- 
triangular with the base outwards and nearly straight, the apex inwards and 
a little forwards, rounded off, to which the anterior and posterior sides con- 
verge in curved lines; the grinding surface supports three principal tubercles 
or cusps, two on the outer and one on the inner side: there are two smaller 
elevations, with a depression on the summit of each, situated in the middle of 
the crown, and the whole is surrounded with a ridge, which is developed into 
a small cusp at the anterior and external angle of the tooth. These teeth 
form the principal difference between the dentition of the present genus and 
that of the Chceropotamus, in which the corresponding false molars are rela- 
tively smaller and of a simpler construction, having only a single external 
pyramidal cusp, with an internal transverse ridge or talon at its base. The 
true molars, three in number on each side, closely correspond in structure 
with those of the Cheeropotamus. They present four principal conical tuber- 
cles, situated near the four angles of the quadrilateral grinding surface. 
Each transverse pair of tubercles is connected at the anterior part of their 
base by a ridge, which is raised midway into a smaller conical tubercle with 
an excavated apex. The crown of the tooth is surrounded by a well-marked 
ridge, which is developed, asin the third and fourth false molars, into a sharp- 
pointed cusp at the anterior and external angle of the tooth. The hindmost 
molar is more contracted posteriorly, and its quadrilateral figure less regular 
than the two preceding molars. 
The sockets of the canines or tusks indicate that these teeth were relatively 
as large as in the Peccari, and that they were directed downwards. The 
temporal muscles were as well-developed as in the Peccari, the depressed sur- 
face for their attachment extending on each side of the cranium as far as the 
sagittal suture. 
The frontal bones are divided by a continuation of the sagittal suture. The 
nasal suture runs transversely across the cranium parallel with the anterior 
boundary of the orbits. The lachrymal bone reaches a very little way upon 
the face. The external angle of the base of the nasal bone, which is of con- 
siderable breadth, joins the lachrymal, and separates the superior maxillary 
from the frontal bone. The anterior margin of the malar bone encroaches a 
little way upon the face at the anterior boundary of the orbit. The external 
aperture of the sub-orbital canal is situated about three-fourths of an inch 
from the anterior boundary of the orbit. The under surface of the palatal 
processes of the maxillary bones is rugose, as in the Peceari; the portion of 
the skull, including the intermaxillary bones and the incisive teeth, is unluckily 
broken off and lost. 
That the eye was full and large, is indicated by the size of the optic foramen 
and the capacity of the orbit, the vertical diameter of which equals 1 inch. 
The upper part of the cranium, anterior to the sagittal suture, is slightly 
convex from side to side ; its longitudinal contour is nearly straight. The face 
gradually becomes narrower anteriorly ; it is slightly concave at the sides. 
The general form of the skull was probably intermediate in character be- 
tween that of the Hog and the Myraz. The large size of the eye must have 
given to the physiognomy of the living animal a resemblance to that of the 
Hare and other timid Rodentia. 
Without intending to imply that the present small extinct Pachyderm was 
more closely allied to the Hyrax than as being a member of the same order, 
and similar in size, I have proposed to call the new genus which it unques- 
tionably indicates, Hyracotherium, with the specific name leporinum. 
In the eocene sand underlying the red crag at Kingston or Kyson in 
a2 
