486 Zaolugical Society. 



The direction of the incisors in the Toxodon differs very little from 

 that which v/e find in many of the typical Perissodactyla, and the 

 absence of roots is simply a physiological adaptation, and an indu- 

 bitable proof that the detrition to which they were subjected was 

 considerable ; while on the other hand, the whole structure of the 

 cranium is on the ungulate type, especially different from the Capy- 

 bara and the forms allied to it, whose skulls present so many striking 

 characters, that if any resemblance really did exist, an anatomist to 

 whom they were familiar would certainly perceive it at a glance. 



It is a matter of considerable regret to me, that before concluding 

 my notice of the Perissodactyla, I am again compelled to differ from 

 that high authority to whom we owe so much, and in whose footsteps 

 I may here be said, as it were, to follow. Although I am prepared 

 to show that the evidence of the teeth, on which Prof. Owen decided 

 the place of his genus Hyracotherium, is not so strong as it may 

 appear ; yet, on the other hand, their resemblance to those of the 

 group to which I must transfer it is not so striking as to have caused 

 me in the least to doubt the correctness of the place assigned to it, 

 until I was well satisfied of the value of the cranial characters which 

 I have pointed out. Although the true molars resemble those of 

 the Choeropotamus and other non-ruminant Artiodactyla in the tuber- 

 cular form of the four principal eminences, and in having the ridge 

 surrounding the base more complete than is usual in the Perisso- 

 dactyla, yet to make the resemblance good, they should have, in 

 addition to the two smaller tubercles, the one in the front, the other 

 in the middle of the tooth, a third one behind ; and the fact is well 

 worthy of attention, that each of these secondary tubercles is placed 

 upon the angle of a bent ridge which connects the pair of larger ones 

 immediately behind it, and which in the smaller species {Hyracothe- 

 rium Cnniculus) exists, while the little tubercle itself is wanting ; thus 

 showing that the ridge is a more essential part of the tooth than the 

 tubercle developed upon it ; and this ridge just marks out in a rudi- 

 mental way the bent transverse ridges in the Rhinoceros, Tapir, Pa- 

 Iseotherium, and other allied genera. The two last premolars differ 

 from the true molars only in the non-development of the inner tubercle 

 of the posterior pair, but of which a slight rudiment is still traceable ; 

 and the sudden change of form between these teeth and the two first 

 is met with in no other genus, either of the Artiodactyle or Perisso- 

 dactyle group. This would be perfectly in accordance with law, if 

 the third and fourth molars belonged to the milk series, and the ani- 

 mal were Artiodactyle ; but the whole series has the appearance of 

 adult completeness, and neither the form nor the degree of wear of 

 these teeth at all indicates such to be their nature ; — indeed Prof. 

 Owen himself never once hints at such an idea. To whichever group, 

 then, this little animal be referred, the teeth will present marked ex- 

 ceptional characters, and therefore it becomes more necessary to seek 

 for further evidence. I was first led to suspect a Perissodactyle affi- 

 nity, through observing, by the figures and description published in 

 Prof. Owen's very useful work on the British Fossil MammaUa, that 

 the nasal bones exhibit the character of this group in a very decided 



