Farr.l 150 [May 15, 



The anterior border of the mandible is rounded and tlie teeth are ar- 

 ranged in the segment of a circle. 



The fourth lower premolar is wider transversely than any of the other 

 teeth, while the posterior half of Pm. 3 is wider proportionately than any 

 of the remaining teeth, with the exception of the former. Pm. 4 has a 

 massiveness not seen in the other lower teeth. Sometimes this is so 

 marked that if the teeth were not found together they would in all 

 probability be attributed to a larger individual. It had long been sup- 

 posed that the superior incisors were not pitted. Prof. Scott * has sepa- 

 rated Mesohippus from Miohippus on the character of the upper incisors. 

 A skull in the Princeton collection shows the upper incisors which seem 

 to be pitted, but as they are so much worn a determination of their 

 character is not possible. Osborn and Wortmau f have just described 

 these teeth and through the kindness of these gentlemen I have been 

 permitted to examine this beautifully preserved skull. The two outer 

 pairs of incisors show a distinct invagination, which is not, however, 

 present on I. 1. Upper Pm. 1 is a small single-coned tooth, which has 

 two distinct roots. The ciugulum is well-developed on the inner side, 

 enclosing a deep pocket. Anteriorly there is a tiny accessory ccnule. 

 The corresponding tooth of the lower jaw is very small and inserted by 

 only a single fang. Pm. 4 of the upper series is wider transversely than 

 any of the other teeth. 



Succession of the Teeth. 



From all that can be observed the three large deciduous molars first 

 appear simultaneously in both jaws. The next tooth to appear is that 

 Avhich represents Pm. 1 of the permanent set. Nothing is known as to 

 the time of appearance of the incisors and canines, but judging from 

 analogy we may presume that they appear as early as the milk molars. 

 A mandible of Mesohippus (No. 11107), with milk dentition and M. 1 

 of the permanent set, shows alveoli for the three incisors and c. :ne. 

 The next tooth to appear (after persistent Pm. 1) is M. 1, which is suc- 

 ceeded by M. 3. Next the temporary molars are replaced by the perma- 

 nent premolars. 



In the upper jaw these are replaced in the following order: Pm. 4, Pm. 3, 

 Pm. 2 (Pm. 1 persisting in both jaws). One specimen shows Pm. 4 almost 

 ready to erupt, while Pm. 3 is very much smaller and the germ of Pm. 2 is 

 very feebly developed. The mode of succession in the lower jaw seems 

 to follow the same order. In specimen No. 10995, M. 1 and M. 2 have ap- 

 peared, and the germs of the permanent teeth are seen by picking away 

 the bone and exposing roots of teeth, where the germ of Pm. 4 is seen to 

 be better developed than that of Pm. 3. This also accords with the rate 



* Ti-ann. Amcr. Phil. Soc, 1883, p. 79. In the light of present knowledge it seems be.st 

 to abandon tlie genus Miohippu.s and to make the genus Mesohippus include the John 

 Day equines as well as tlie White Kiver forms. 



t IhiU. Am. Mas. Nat. Hist., Vol. vii, p. o53. 



