270 S. W. WILLISTON NEW GENERA OF PERMIAN VERTEBRATES 



Humerus (plate 11, figures 2-5). — The humerus is of the usual tem- 

 nospondyl form, differing from that of Eryops and Trematops especially 

 in the absence of the ectepicondylar process, so conspicuous in the former 

 and in Euchirosaurus. It is broadly expanded at either extremity in 

 planes nearly at right angles with each other, and has a rather slender 

 shaft in the middle. The lateral or radial process is very stout; the 

 medial or ulnar process is indicated by a slight ridge or elevation just 

 below and in front of the inner extremity of the proximal articular sur- 

 face. The digital fossa on the inner side is rather shallow and broad. 

 The short shaft is subcylindrical in cross-section, with a sharp ridge run- 

 ning from the outer side of the lateral process to terminate in the supi- 

 nator ridge. The capitular surface for the radius is subhemispherical in 

 shape, looking mainly forward. The trochlear surface for the ulna is 

 small. The inner, condylar border is moderately dilated and relatively 

 thin; the ectepicondylar or supinator border thick. There is no indica-- 

 tion whatever of the ectepicondylar process below the lateral process on 

 the outer side. 



Humeri of allied forms. — Among the early land vertebrates no part of 

 the skeleton is more characteristic than the humerus. As a rule, the humeri 

 of the amphibia and reptiles resemble each other markedly — rather stout 

 and short bones, with the extremities greatly expanded and their planes 

 divergent from each other in an angle of from 60 to 90 degrees. Though 

 usually thickset and short, in some of the reptiles it is as slender as that 

 of existing lizards, an example of which is seen in the humerus of Pleuris- 

 tion, a small cotylosaurian reptile — a little smaller than the existing 

 Sphenodon. Climbing reptiles have slender humeri, fossorial and aquatic 

 reptiles thickset and short humeri, though in some amphibious forms, 

 such as the crocodiles, it is rather slender. In strictly terrestrial, cur- 

 sorial reptiles it is never short and stout, from which it may be inferred 

 that the amphibians and reptiles from Texas with such thickset humeri 

 are either fossorial or aquatic. But it is an inference only, since there 

 may be other causes to account for the robustness, of which we are not 

 yet aware. One conclusion is, however, justifiable: animals with such 

 humeri as are shown in plate 15, figures 4 and 5, were certainly not 

 quick running in habit ! 



As a very general rule, the humeri of amphibians and reptiles may 

 be distinguished by the absence of an epicondylar foramen in the 

 former; its frequent presence in the latter. Among living reptiles the 

 crocodilia have neither; Sphenodon has both ectepicondylar and entepi- 

 condylar foramina. Lizards frequently have an ectepicondylar foramen 

 or groove; even the niosasaurs possess such a groove, though no record of 



