MUSEUM OF COMPARATIVE ZOOLOGY. iat 
this skull with that of H. nebrascense, we find about the same relative 
height as between the skulls of Metamynodon and Amynodon. The antero- 
external column is less sharply folded than in H. nebrascense ; the first molar ° 
has a small conical tubercle at the entrance of the valley; the outer wall of 
the last molar extends beyond the posterior crest much more than in the 
other species of the genus, as in Amynodon and the Lophiodontide. The 
transverse crests are subequal and extend obliquely across the crown, thus dif- 
fering from H. arcidens, Cope, in which the anterior crest is the longer and 
curves around the posterior. 
The skull, which was that of a young animal, Jacks most of the facial por- 
tion. The extreme breadth across the zygomatic arches is 6} inches, while 
the height of the occiput is 24 inches ; in a young specimen of H. nebrascense 
these measurements are 43 and 3 inches, a great difference of proportions. 
The periotic is exposed on the surface of the skull; the parietals are short; 
the post-glenoid and post-tympanic processes are separated below; the lachry- 
mals extend considerably on the face. 
MEASUREMENTS. . plasiceys. HH, BEDE 
Upper molar series, length . . ...... 103 070 
Rirstmolir widiit . 0. on «es? o Te Che 2085 .026 
Second “ ee eek ee cw) 8 2) lat oe, ee en .028 
Timed: ““ TS emia a, ee rer) Ty ls ae 97 026 
second molar, letigth yo. & 2 . 2. < « « s °.035 .027 
Third s o Pe. ee aera aon ei!) = 
The proportions of the teeth thus differ considerably in the two species. In 
H. planiceps they increase in size from m. 1 to m. 3, while in the smaller ani- 
mal m. 2 is the largest. In the former species the molars closely resemble 
those of Amynodon, but differ widely from the proportions found in Metamy- 
modon. In fact, this animal may turn out to belong to a genus very different 
from Hyracodon, but at present we prefer to retain it provisionally in that 
group. 
ANCHITHERIDA. 
Mesohippus (Anchitherium) Bairdi, Leidy. The genus Mesohippus, 
Marsh, differs from Anchitherium in the structure of the incisor teeth, which 
have no enamel pit. The Cambridge collection contains an excellent skull 
and brain cast, the description of which is reserved for the memoir. 
