464 GEOLOGICAL SUEVEY OF THE TEKEITOEIES. 



tively sliort head, and with the uninterrupted series of teeth which 

 belongs to all the oldest forms of the mammals and to the higher qua- 

 drumana. 



The early relations of the camels is a question, heretofore very obscure, 

 which has been greatly elucidated by the researches in Colorado during 

 the present season. These ruminants differ from all of their order of 

 quadrupeds in having one incisor-tooth in the upper jaw on each side, 

 and in a remarkable structure of the neck- vertebrae. In the latter, the 

 artery that conveys blood to the brain in part occupies the vertebral 

 canal with the nervous cord. In other ruminants, as in most mammalia, 

 it is carried by the lateral process of the vertebrae in a tube at their 

 bases. 



Camels and llamas have a limited number of premolar teeth of the per- 

 manent series, and a larger number in the milk-series ; the excess not 

 being replaced when shed. They have, at a very early stage of develop- 

 ment, indications of a full series of upper incisors, which are early 

 absorbed. The extinct camels {Frocamelus) of the latest of our Western 

 Tertiary formations, supposed to be the Pliocene, have been shown by 

 Leidy to retain, in their permanent dentition, the full number present in 

 the milk-series, thus resembliug the younger stage of the modern camels 

 and llamas rather than the adult. I have also found that these early 

 camels j)ossessed a full development of the character seen in the foetus 

 of various ruminants. In this respect, the Frocamelus resembled their 

 still earlier predecessors. It is also evident that the change from Fro- 

 camelus to Camelus may be explained by a process of retardation of the 

 growth of the teeth. 



The ruminants called Tragulidw are now confined to the warm regions 

 of Asia and Africa, but they were formerly widely distributed over the 

 earth, especially during the Miocene period. They embraced then, as 

 they do now, some of the smallest and most elegant of the cloven-footed 

 Ungulata. In France, three genera have been discovered, which em- 

 brace numerous species. The most numerously-represented genus is the* 

 Amphitragulus. In North America, four genera have been found in 

 corresponding formations, representing five species. Two of them be- 

 long to Sypertragulus ; while Hypisodus includes the smallest of the 

 known species, the H. minimus, Cope, which was not heavier than a cat- 

 squirrel. In these musks, the first premolar teeth have a peculiar position, 

 being more or less approximated to the incisor-teeth. The Leptomeryx 

 evansii, Leidy, is a species of medium size, which has the i3ermauent 

 I)remolar teeth of the same form as the milk-premolars ; while in Hyper- 

 traguluSj the permanent premolars take on quite a different form, thus 

 making a step in advance not attained by the former. It has been 

 ascertained by me that Fobbrotlierium, has the peculiar neck- vertebrae 

 that belong to the camels, and also similar resemblances in the forefoot, 

 differing in both respects from TraguUdm. Like the camels, it has only 

 two toes, while Tragulidw have four ; but then it is like the latter in 

 having these toes entirely separate, as in a hog, and not united into the 

 common bone of ruminants. The conclusion is that Foebrotherium is the 

 prototype of the camels, and that it is near the common ancestor from 

 which TraguUdce seem to have branched off. This ancestor undoubtedly 

 was nearly related to certain fossil ungulates found in the Eocene of 

 France. 



INSECTIVORA. 



Numerous species of this order were discovered during the explora- 

 tion of the Colorado Miocene. Two species only had been previously 

 known in the formation, namely, the Leptictis haydenii and Ictops daJco- 



