TRIMERORHACHIS, A PERMIAN AMPHIBIAN 



249 



palate, like that of the reptiles; T rimer orhachis has very wide 

 parasphenoidal vacuities, though only a slender parasphenoid. 

 Certainly the resemblances between Trimerorhachis and Diplo- 

 caulus in the eyes, clavicular girdle, and small limbs are adaptive, 

 not genetic, for Diplocaulus is a holospondylous amphibian. 



In the evolution of the feet we know that aquatic adaptation 

 has frequently resulted in the more or less complete chondrification 

 of the mesopodials. One need only to study the progressive evo- 

 lution of the mosasaurian paddle to be convinced of this/ And 

 the cetaceans are still better examples. 



Fig. 



sp 



-Trimerorhachis insignis, skull, from the side. Explanations as in Fig. i 



In the progressive adaptation of the tarsus to terrestrial life, 

 there has been a continued loss of elements and a strengthening and 

 closer articulation of those remaining, not only in the mammals, 

 but in the reptiles as well. In the most primitive known tarsus, 

 that of Trematops,^ there are not less than thirteen ossified bones, 

 four in the first row, four in the second, and five in the third. The 

 early amphibian was a truly crawling animal, dragging its body 

 over the surface of the ground. Its feet were directed outwardly, 

 and the motion of the tarsus on the leg was chiefly lateral; the 

 angle between the over-extended foot and the leg was always 

 obtuse. In the evolution of the reptiles greater speed was attained 

 by the elevation of the body from the ground in locomotion. No 

 modern reptiles crawl, in the strict sense of the word, except the 

 snakes and legless lizards— for the most part at least; many even 



^ WiUiston, American Permian Vertebrates, p. 45. 



^ Williston and Case, Carnegie Publication, No. 181 (1913), p. 56. 



