292 



ON THE EXTINCT MAMMALIA OF 



presents a beak or process at the fore part, as if indicating an approaching association 

 of the column with the antero-median column. The measurements of both specimens 

 are as follow : 



Lines. 



Length externally, ........ 8 



Length internally, ........ 5 



Antero-posterior diameter, . . . . . . .10 



Transverse diameter, . . . . . . .6 



Breadth of internal column, . . . . . .3 



MERYCHIPPUS. 



So far as can be ascertained from the material at command, the molar teeth of this 

 genus are characterized in the same manner as those of Proiohij^j^us ; hut while 

 the latter has the face in advance of the orbit constructed on the same plan as in 

 Equus, in MerycJiippus it exhibits in a corresponding position a remarkable depression 

 or fossa. In comparison with the temporary molars of Equus, those of Merychippus 

 approach more nearly in their appearance the teeth of the second family of the 

 Solidungula, — that is to say, the Anchitheridce. 



The genus MerycMppus was originally proposed in the Proceedings of this Academy 

 for 1856, page 311, on a small fragment of an upper jaw containing two teeth, of a 

 supposed equine animal, discovered by Dr. Hayden at Bijou Hill. 



The teeth, represented in figures 3, 4, plate XVII, consist of the first and second 

 superior molars, apparently of the temporary series, though the jaw fragment filled 

 with a calcai'eous matrix, in its present condition, shows no evident signs of occu- 

 pancy by permanent successors. In their mode of insertion and general appearance 

 they bear some resemblance to the upper true molars of the Deer, but depart from 

 them in many important anatomical characters. 



In general form, proportions, and mode of insertion, the teeth also resemble those 

 of Anchitherium, with which Merychippus was nearly allied. The crowns are com- 

 posed of six lobes, as in Anchiiherium, and the intervals in the fossil are nearly 

 devoid of cementum, which was supposed to be the natural condition of the teeth, 

 until the discovery of other specimens led to a different conclusion. The fossil is 

 somewhat weathered, and appears to have accidentally lost its cementum, though 

 small portions still remain in the narrowest interspaces of the lobes, and in the 

 second molar it fills up the vacuity between the postero-median and postero-internal 

 lobe. The summits of the lobes are worn so as to exhibit nearly continuous tracts of 

 dentine, as represented in figure 4. The fore part of the anterior molar has been 

 lost, but otherwise the teeth are nearly perfect. 



The external lobes of the crown of the molars in question have the same form. 



