296 ON THE EXTINCT MAMMALIA OP 



upon outwardly by the position of the fourth molar tooth, and would have been 

 rendered deeper after further protrusion of the latter. The orifice of the infra-orbital 

 canal opens just in advance of the ant-orbital fossa, over the position of the fore part 

 of the last temporary molar tooth. 



At the upper angle of the premaxillary fossa the lachrymal suture is observable, 

 and in advance of this the upper border of the specimen, to its anterior extremity, is 

 defined by the nasal suture. 



Having thus attempted to establish the generic identity of the Bijou and Niobrara 

 fossils above described, I shall proceed to give an account of additional fossils, appa- 

 rently referable to the two species oi Merycfdppus, under the respective head of each 

 of the latter. 



Mertchippus insignis. 



As stated in the notice of the genus, the species Merychippus insignis was first 

 indicated by a fragment of a jaw containing two teeth, from Bijou Hill. The Nio- 

 brara collection of fossils contains a number of specimens apparently referable to the 

 same species, as follow : 



1. An entire series of superior permanent molar teeth, contained in an alveolar 

 fragment of the jaw. The teeth appear to be about two-thirds worn away, and are 

 inserted into the jaw by the fangs alone. The triturating surfaces, represented in 

 figure 5, plate XVII, have much such an appearance as would be exhibited in the 

 teeth oi Protohippus perditus at the same age or condition of abrasion. 



The transverse diameter of the teeth is greater than that from before backward, 

 except in the first of the series, but this disproportion is mainly due to their much 

 worn condition. The central crescentoid lakes of the triturating surfaces are of 

 simple character, contracted, and have their contiguous horns in general much pro- 

 longed outwardly in comparison with the distal ones. The inner columns of the 

 crown are circular in transverse section, and they join the median lobes hy wide 

 isthmi. 



In all except the back two teeth, the postero-internal column has lost its distinct 

 character by an obliteration of the posterior inflection or valley of the crown. The 

 bottom of the valley remains in the first molar as a small circular islet, and in the 

 third molar as a minute ring. It is totally obliterated in the second and fourth 

 molars. 



In the fourth molar, the antero-internal column in some degree has lost its distinct- 

 ness by closure of the outlet of the valley between it and the column behind. The 

 bottom of the valley is left on the triturating surface as an oblique ellipsoidal islet. 

 The other molars present an appearance indicating that they would have assumed 

 the same condition as that just described, at a later stage of abrasion. 



