MUSEUM OF COMPARATIVE ZOOLOGY. 171 



this skull with that of H. nehrascense, 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. nehrascense ; 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 Lophiodontidce. The 

 transverse crests are subequal and extend obliquely across the crown, thus dif- 

 fering from JI. arcidens, Cope, in which the anterior crest is the longer and 

 curves around the posterior. 



The skull, which was that of a young animal, lacks most of the facial por- 

 tion. The extreme breadth across the zygomatic arches is 6^ inches, while 

 the height of the occiput is 2^ inches ; in a young specimen of H. nehrascense 

 these measurements are 4| 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. K planiceps. H. nebrascense. 



M. M. 



Upper molar series, length 103 .070 



First molar, width 035 .026 



Second " " 036 .028 



Third " " 037 .026 



Second molar, length 035 .027 



Third " " , 040 — 



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 srnaller 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- 

 nodon. 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. 



ANCHITHERID^. 



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. 



