NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 121 



resemblances between the South African and the American forms, he beUeves 

 that it is possible to demonstrate a well-established tendency in their evolu- 

 tion and well-established stages of evolution in different lines of these reptiles, 

 and he believes that this is the essential point in any true classification, which 

 must consider the phylogeny of a group. 



Broom has shown and insisted upon the important tendency in the evolu- 

 tion of the South African forms toward the lengthening of the limbs, the 

 strengthening of the tarsus and carpus, a reduction of the digital formula to 

 2. 3, 3, 3, 3, the change in the pelvis and scapula, the reduction in the size 

 of the bones in the articular region of the jaw, and the enlargement of the 

 squamosal with a coincident reduction of the quadrate. He says: "" 



"The Pelycosaurs were slow-moving crawlers with short, lizard-like limbs; the 

 Therocephalians were active runners, with mammal-like limbs. The structure in 

 the limb girdles is in harmony with the diflferences in the limbs." 



Again:'' 



' ' If this conclusion be correct, we may regard the American and South African 

 Permian faunas as derived from a common origin, but having evolved in quite 

 different directions. The American types undergo many curious specializations. 

 The African, or more preferably the South Atlantic type, is chiefly remarkable for 

 the great development of the limbs. The Pareiasaurians, the Dinocephalians, the 

 Therocephalians and the Anomodonts have all developed powerful limbs, and not 

 improbably all independent of each other. What may have been the cause we can 

 not at present tell, but it was a most fortunate thing for the world. It was the 

 lengthened limb which gave the start to the mammals. When the Therapsida took 

 to walking with its feet underneath, and the body off the ground, it first became 

 possible for it to become a warm-blooded animal. ' ' 



None of these points would Williston dispute. If he were to simply insist 

 that the South African and the American Permo-Carboniferous reptiles were 

 a heterogeneous group, with many distinct specializations, but retaining cer- 

 tain primitive characters upon which the specializations were based, there 

 could be no contention, but to fail to fully weigh the manifested destiny of 

 radically different groups seems to the writer too conservative to express the 

 state of our knowledge. While agreeing with Williston in the futility of 

 attempting a detailed classification, the author desires to express his opinion 

 of the situation as follows: 



(I) American Permo-Carboniferous Therocrotaphic reptiles (Pelyco- 

 sauria). With primitive characters allying them to the African Therapsida, 

 and pointing to a common ancestor, but with all their specializations upon 

 a reptilian theme, and leading to extinction, or higher reptiles. 



(II) South African Permian Therocrotaphic reptiles (Therapsida). With 

 primitive characters allying them to the American Pelycosaurs, and pointing 

 to a common ancestor, but with all their specializations upon an advancing 

 theme, and leading to extinction, or to the Mammalia. 



» Broom, Trans. Roy. Soc. S. Af., vol. i, 1910, art. 2. 



•• Broom, Bull. Amer. Mus. Nat. Hist., vol. 28, art. 29, p. 234, 1910. 



