264 [Sept. 1847. 



there are ten specimens of permanent molars, one a superior posterior molar of 

 the left side, and five inferior molars of the right side, and four of the left side. 

 These were all obtained from that celebrated fossil bone deposit, Big-Bone 

 Lick, Kentucky, where they were associated with the Mcgalonyx, Mastodon, &c, 

 and are a part of a donation to the Academy by Mr. J. P. Wetherill. The ex- 

 ternal cementum is almost entirely removed, and the color, which is brown in 

 the inferior molars, a bluish black in the superior molars, corresponds with 

 that of their fossil associates. They are very little inferior in size, both in 

 length and diameter, to the corresponding teeth of the recent E. caballus. The 

 lateral diameter of the inferior molars hardly varies at all, the difference exist- 

 ing in the transverse diameter, which gives to the teeth a rather more com- 

 pressed appearance. The superior posterior molar tooth in all species of Equus 

 is much curved, so that but little difference is observable in this respect in the 

 fossil specimen. The bodies of the inferior molars are considerably more curved 

 laterally than is usual in the corresponding teeth of the recent horse, which 

 fact, however, was not to be expected from the greater degree of curvature in 

 the superior molars. 



The enamel folds generally are more delicate, but I do not find sufficient pe- 

 culiarity in their course to render them characteristic. On comparing the 

 crowns of these fossil molar teeth, with the recent species, I find a remarkable 

 degree of resemblance to exist, and in fact, greater differences may be found in 

 this respect, in different individuals of the existing species. The posterior part 

 of the enamel folding of the posterior tooth is rather narrower, and has a deeper 

 groove upon the outer side than I have seen in the recent tooth. The supe- 

 rior molars lead to more positive results than the inferior, yet it is necessary to 

 be very careful, for if we do perceive more differences in these particular teeth 

 in different species, than exists in the inferior teeth, so also do we find a 

 greater variation among them in different individuals of the same species. This 

 variation in the same species is very striking in the case of the posterior tooth 

 of the recent horse, as may be seen by comparing any number of specimens. 

 In this particular tooth in the recent horse, there is always a disposition to the 

 formation of a third isolated enamel fold, always small and posterior to the 

 others. Sometimes it appears as if the disposition existed, but for want of 

 room in the process of development of the tooth, the ordinary posterior, isola- 

 ted enamel fold becomes united by an isthmus to the peripheral fold. In the 

 fossil tooth no disposition of the kind has existed, so that it has more the ap- 

 pearance of the other molars, and indicates a less amount of room for develop- 

 ment, and consequently a smaller jaw. 



From the foregoing description it will be perceived that I have fixed upon no 

 absolute characters for determining this species with any degree of accuracy, 

 and that this is not possible, I may state upon the authority of Cuvier, who ac- 

 knowledged his incompetency to find characters, " assez fixes," to pronounce 

 upon any species of horse, examined by him, from an isolated bone,* and it is 

 therefore only from their being fossil American teeth coinciding with the E. 



*Cuvier, Ossemens Fossiles, 4 Ed. T. 3, p. 216. 



