13^ Bulletin American Museum of Natural History. [Vol. X, 



known to speak with any degree of confidence as to its correct- 

 ness. 



The principal changes of structure in the evolution of the 

 group relate largely to the limbs and skull, although, as is 

 well known, the cervical vertebrae are highly characteristic in the 

 whole family. Unfortunately the cervicals are entirely unknown 

 in the earliest representative, Frotylopus, but in the succeeding 

 genus, PoebrotJwriuni^ they had already assumed the typical 

 cameloid peculiarities. If one is permitted to judge by analogy, 

 it is more than likely that Frotylopus possessed the peculiar 

 cameloid cervicals, and it seems highly probable that one of the 

 crucial tests of tylopodean affinity, in any genus older than this, 

 will be found in the incipient changes leading to this modifica- 

 tion. 



The changes in the limbs consisted mainly in (i) their gradual 

 elongation, (2) coossification of certain bones, (3) modification of 

 the patella and loss of the greater part of the tibia, and (4) the 

 subtraction of digits, the change in the character of the feet and 

 modification of the phalanges, together with the coossification of 

 the i)odial elements into a cannon bone. In this connection 

 should also be mentioned the development of a double bicipital 

 groove upon the humerus and the modification of the carpal and 

 tarsal elements. 



The question of the elongation of the limbs is closely asso- 

 ciated with the general increase in size, in which there is as 

 complete a gradation, from the little Frotylopus, scarcely larger 

 than a good-sized Jack Rabbit, to the more modern species, 

 larger than the Dromedary, as the most hypercritical opponent of 

 the Evolution Theory could possibly demand. In the matter of 

 the coossification of certain bones, the ulna and radius were the 

 first to be affected by this process. As we have already seen, it 

 had taken place in Frotylopus in a very old indli'nlual, and then 

 only in the middle of the shaft, leaving the proximal ends entirely 

 ununited. In Poehrothcriuiii the complete coossification of these 

 elements was accomplished in young individuals before the milk 

 dentition had been completely shed, and before the epiphyses had 

 united to the shafts of the long bones, as is demonstrated by many 

 specimens in the Museum collection. In all tlie later types these 

 bones are firmlv united. 



