No. 2300. 



SKELETON OF DIMETRODON GIGA8—GILM0RE 



537 



illustrated in figure 8 must await the discovery of a more complete 

 articulated manus. 



The carpus of Dimetrodon as known at this time consists of 11 

 elements, though Williston is of the opinion that a pisiform ^ will 

 be foimd articulating in the interval between the ulna and ulnare, 

 as in modern reptiles and as it does in Varanosaurus , Casea, 

 Limnosceles , and Trispondylus. 



The proximal row consists of three bones (the radiale, intermedium, 

 and ulnare), a median row of two centrale, and a distal row of five 

 carpale. These carpal bones articulate with one another by well- 

 defined facets and with the metacarpals by equally well-developed 

 articular faces, which indi- 



cate clearly the spreading 

 nature of the digits and 

 especially the decided di- 

 vergence of digit 5. 



The radiale is a stout 

 block-like bone with a flat- 

 tened proximal end which 

 articulates with the 

 radius. The distal end is 

 wedge-shaped, formed by 

 two nearly equal but 

 oblique articular faces. 

 The outer face meets cen- 

 trale 2 (Cg), the inner ar- 

 ticulates with centrale 1 

 (Ci). The posterior side 

 of the radiale is channeled 

 by a deep vertical groove 

 running from top to the 

 bottom. 



Theulnare is a relatively 

 thin but elongate element, broadly grooved vertically on the posterior 

 side. The subconvex articular proximal end forms the chief articu- 

 lation of the foot with the ulna. The flattened distal end unites 

 with carpale 4 and 5; most extensively with the former. On the 

 radial side are two distinct articular facets — the upper for the inter- 

 medium, the lower unites with centrale 1. The rounded border be- 

 tween these two facets represent the ulnare boundary of the very large 

 intracarpal foramen such as is found in S'phenodon. This is consid- 

 ered by Osborn to be a "primitive feature' characteristic of other 

 Diapsida." 



FiQ. 8.— Left fore foot of Dimetrodon gigas Cope, No. 

 8635, U.S.N.M. One-third natural size. Ci, Ci, cen- 

 trale 1 AND 2; /, INTRACARPAL FORAMEN; in. INTERMEDIUM ; 



ra, eadlale; S, sesamoid; ul, ulnare; 1, 2, 3, 4, and 5 



CARPALE 1 TO 5; I, U, 111, IV, V, DIGITS 1 TO 5. 



I Williston, S. W. American Permian Vertebrates, 1911, p. 103. 

 s Bull. Amer. Mus. Nat. Hist., vol. 23, p. 269, 1907. 



