340 Prof. Owen—On a New Genus of Mammal. 
‘cingulum,’ resemble those parts in Hyracotherium and Pliolophus, 
but are more irregular and wrinkled. Besides the original speci- 
men of the upper jaw of Hyracotherium leporinum, ficured i in my 
‘ British Fossil Mammals,’ p. 422, figs. 166, 167, I have mec enabled 
to strengthen the results of the above comparison, by extending them 
to a second specimen of that rare genus and species, from the 
London Clay at Herne Bay, now inthe British Museum. This speci- 
men consists chiefly of the maxillary part of the skull, and includes, 
m1, 2, 3, p 4, and p 3, of the right side; m2, m 1, p 4, p3, and part of 
the crown of p 2, of the left side: it is from an older animal than the 
one first described, the crowns of the molars being worn down to the 
cingulum; and, as a tract of dentine is exposed continuously along the 
connecting ridges to the inner cusps, the resemblance to the pattern 
of grinding surface in the genus Lophiodon, fig. 5, is made greater. 
In this specimen of Hyracotherium, fig. 2, I caused the matrix to 
be removed with great care from the bony palate, and believe that 
the posterior margin describes the notch extending as far forward 
as the interval between m 1 and m 2. <A pair of canals open 
obliquely forward upon the bony palate opposite the same in- 
terval. 
In Miolophus it appears that the bony palate extended backward 
beyond the molar series. But amore certain osteological difference 
from Hyracotherium is shown in the relative position of the anterior 
piers of the zygomatic arches, the posterior border being on the 
transverse parallel of the fore part of m 2 in Miolophus, fig. 1, where- 
as in Hyracotherium it is on that of the hind part of m 3, fig. 2. 
The above-defined differences in the pattern of the grinding surface 
of the molar teeth are associated with a more triangular, or less 
quadrate, shape of that surface in Mivlophus, and a minor degree of 
difference between the molars and premolars. 
In Lophiodon, the grinding surface of the upper molars, m 1 and 
m 2(Pl. X., fig. 5), has two outer lobes or cusps, a, 6, connected with 
two inner lobes, c, d, by ridges proceeding from the fore part of the 
base of each outer lobe to the apex of each inner lobe, and inclining 
obliquely backward: the cingulum, m, girts the fore, inner, and hind 
part of the crown, subsides at the base of the antero-external lobe, a, 
and developes a small cone or cusp, 7, at the antero-external angle. 
In Pliolophus this pattern of grinding surface is manifest as 
follows :—the two outer lobes, fig. 4, a, 6, are rather rounded than 
three-sided cones: the connecting ridges proceeding from the fore 
part of their base are low, save at their inner part, which rises into 
asmall cone ; they are chiefly represented by the three cones e, f; 
the two inner lobes, c,d, are rounded cones, more independent than 
in Lophiodon, by reason of the partial subsidence of the connecting 
ridges. The cingulum in Pliolophus closely corresponds with that 
of Lophiodon in the place of its interruption and of its chief develop- 
ment. 
By the increase of the connecting ridges at the cost of the ter- 
minal lobes, the bilophodont type of tooth is acquired, as in Tapirus 
and Dinotherium. By the increase of the outer and inner pairs of 
