204 ON THE EXTINCT MAMMALIA OF 



This also retains part of an alveolus of the tooth in advance, indicating it to have 

 been in aiDposition with the others. If the reference of the two premolars is correct, 

 it follows that the molar series of Hyopotamus form a continuous row, differing in 

 this respect from Anthracothermm, in which the first premolar is removed from the 

 others by a considerable interval. It is further probable, under these circumstances, 

 that the fourth or last permanent premolar of HijoiMam'm is like the third, as in 

 Anthracoiherlum. On the other hand, if the teeth we have supposed to be the second 

 and third premolars be viewed as the third and fourth, it would then appear as if the 

 second would resemble the third, and both be like the second one of Anthracotlierium. 

 We have not the means of positively determining this question, for the most com- 

 plete of the specimens upon which Prof. Owen founded the genus Hi/opotamtis was 

 the jaw of a young animal, containing in series the permanent true molars and two 

 temporary molars in advance. 



The third upper 2:)reraolar tooth of Hyopotamus americanus, represented in figures 

 3, 4, to the left, is inserted by two fangs, one internal to the other. The crown is 

 half as wide again transversely as it is antero- posteriorly or longitudinally. It is 

 composed of a transverse pair of lobes, as in the posterior pair of premolars of Anthra- 

 cotherium, and as is the case in the upper premolars of ordinary ruminants. The 

 external lobe resembles the corresponding ones of the true molars, but is slightly 

 less in size, and it has its basal angles proportionately less prominent exter- 

 nally. It is worn at the summit and along the anterior border, so as to exhibit a 

 narrow tract of exposed dentine. The internal lobe is considerably smaller and more 

 regularly conical than the other, and is slightly more anterior than posterior in its 

 relation with it. It presents a sub-acute ridge antero-externally joining the base of 

 the outer lobe, and a similar ridge postero-internally continuous as a basal ridge, 

 joining the contiguous portion of the base of the outer lobe. A thick, acute-edged 

 basal ridge bounds the front of the crown and associates the two lobes. The summit 

 of the internal lobe is worn so as to exhibit a central circular spot of dentine. 



The second upper premolar, represented in figures 3, 4, to the right, resembles the 

 corresponding tooth of Auihracotherium magnum. It is inserted in the jaw by three 

 fangs, — two externally, and one postero-internally. The crown is widest in an oppo- 

 site direction to the former teeth, being greatest antero-posteriorly, and is. least in the 

 length. It is trihedral, and is mainly composed of a broad conical lobe, situated 

 obliquely fore and aft in relation with the tooth behind. The broad surfaces are 

 separated by acute ridges, of which the anterior is slightly inflected, and the posterior 

 is reflected and ends at the summit of an abutment formed by the projecting base of 

 the lobe postero-externally. The outer surface of the crown is convex anteriorly and 

 concave posteriorly, with the base prominent and longitudinally corrugated. Inter- 

 nally the crown is bounded by a basal ridge forming a right angle, and including 



