1898.] IVortman, Extinct Camelidce. of North America. 137 



The patella of the later Camels is peculiar in that it is narrow 

 and of great vertical depth. The first evidence of this modifica- 

 tion is seen in the development of a long pointed process upon 

 the inferior border of the bone in Protylopics ; this is continued in 

 Poebrothcrium and Gomphotherium, until in Procaincliis the 

 modern condition is reached. The shaft of the fibula in all the 

 modern types has completely disappeared, and the bone is repre- 

 sented by a distal nodular element which lies under the end of the 

 tibia. In Protylopus a considerable part of the shaft of the bone 

 was present, but it was probably not complete. In Poebrotlicrium 

 the shaft is reduced to a very short bony spicule, and the distal 

 part is partially pushed under the end of the tibia ; in GompJio- 

 therium the shaft has completely disappeared, and in Procamclus 

 the modern arrangement is attained. 



In the matter of the loss of digits I cannot speak with any very 

 great degree of confidence, especially as regards the older types. 

 Our materials fail to reveal the number of toes in the fore feet of 

 Protylopus^ but there is very little doubt that there were four. In 

 one specimen which I have referred to this genus, the lateral toes 

 of the hind feet are reduced to mere nodular splints, while in 

 another specimen in the collection there is evidence that the 

 lateral toes of the hind feet were complete but very slender. In 

 Poebrotherium the lateral toes are reduced to vestiges in both fore 

 and hind feet, while in the later forms even these vestiges 

 disappear. 



The modifications in the character of the feet refer to what may 

 be properly termed a retrograde change, if we regard the subject 

 from the stand[)oint of the podial evolution of the Ungulates 

 generally. Protylopus, as well as Poebrotherium and Gompho- 

 theriu7ti, had apparently a fully developed unguligrade gait. The 

 ungual phalanges are relatively high, keeled upon their dorsal 

 surfaces, and more or less flattened upon their opposed sides. 

 The distal ends of the proximal phalanges do not have their 

 articular surfaces extended upon the dorsal side, while the distal 

 phalanges of the second row have this surface well extended upon 

 the dorsal portion of the bone. This arrangement is highly 

 characteristic of all the higher Artiodactyla, and is the strongest 

 possible evidence that can be adduced from the skeleton of the 

 unguligrade gait. 



