594 S. W. WILLISTON 



orly unusually long. Three of the basal caudals have intercentra, 

 the first chevron appearing between the third and fourth caudals. 

 The three pairs of sacral ribs are turned broadly down at their 

 extremities, the expansion of the first pair nearly equal to the 

 combined extent of the second and third pairs. The ilium (Fig. 

 2, E) has the ordinary form, turned broadly backward, and is wholly 

 without an anterior projection. The humerus (Fig. 2, A, B) is 

 massive and broad, with the distal extremity greatly expanded; 

 the radius and ulna (Fig. 2, C, D) are likewise stout bones. The 

 front feet show the usual structure, so far as the preserved remains 

 enable one to decide; the intermedium is large, the second centrale 

 is ossified, and the pisiform articulates with the distal end of the 

 ulna. The femur (Fig. 2, F) is likewise a stout bone, especially 

 characterized by the low position of the trochanter. 



The genus is removed from the Poliosauridae by the possession 

 of three sacral vertebrae; from the Clepsydropidae by the 

 possession of short dorsal spines, and the different structure of 

 the propodials, as will be seen by comparison of the same parts of 

 Clepsydrops and Dimetrodon. 



THE DEVELOPMENT OF HOLOSPONDYLOUS VERTEBRAE 



In a recent paper^ I discussed the views at present held as to 

 the morphological significance of the rhachitomous pleurocentra 

 and hypocentra in the evolution of vertebrae of the higher forms. 

 The majority of paleontologists believe that the rhachitomous 

 type of vertebra is a primitive one, though there are some, of 

 whom Jaekel is one,^ who deny it. The extraordinary resemblances 

 in nearly all parts of the skeleton between the more specialized 

 temnospondyls and the more generalized reptiles are almost a 

 demonstration of genetic affinity. That we have in any known 

 Permian forms the actual connecting links between the Amphibia 

 and Reptilia is more than doubtful; it is more than probable that 

 annectant forms must be sought for in older rocks, probably those 

 of the lower part of the Pennsylvanian or Upper Carboniferous. 



I Bull. Geol. Soc. Amer., XXI (1910), 265. 



^ Deutsch. geolog. Gesellsch., LVI (1904), 118; Zoologisch. Anzeiger, XXXIV 

 (1909), 200. 



