538 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



Between the radiale and ulnare is the intermedium, which projects 

 well above the proximal ends of those bones, thus interposing itself 

 between and separating the distal ends of the radius and ulna. The 

 inner side of the proximal end articulates with the ulna much as in 

 Sphenodon. Above the flat distal end that articulates exclusively 

 with centrale 1 the sliaft of the intermedium is constricted, but higher 

 it again expands into a thin plate, being terminated by a blimtly 

 pointed proximal extremity. 



On the ulnar side an elongated facet articulates with the idnare. 

 In the present carpus there is no direct contact between the inter- 

 medium and the radiale— a peculiarity which appears to be distinctive 

 of Dimeirodon and which differs from all other Permian reptilia. 

 The illustrations of the carpus as published by Case ^ and by Willis- 

 ton * certainly give the impression that these bones were in contact 

 in Dimetrodon incisivus, but an examination of a carpus of this 

 species shows the same condition prevails as in the specimens here 

 illustrated in figure 8. 



Centrale 1 {C^) has a wedge-shaped proximal end presenting two 

 articular faces; the outer, which is the larger, unites with the radiale; 

 the inner with the intermedium. A slight notch on the ulna side is 

 the contribution of this bone to the boundary of the intracarpal 

 foramen. Below this notch an elongated vertical facet meets the 

 ulnare, while on the opposite side it unites with centrale 2, there being 

 a small foramen at the point where the radiale and the two centrale 

 meet. The distal end is in contact with carpale 3 and 4. 



Centrale 2 {C^) is smaller than C^. Distally it articulates with 

 carpale 1, 2, and slightly with 3. On the radial side there is a dis- 

 tinct articular facet for the "sesamoid," here restored. 



Of the distal row composed of five carpale, carpale 4 is the largest. 

 The first is expanded transversely and presents at its distal extremity 

 a peculiar articulation for metacarpal 1, the external half being on 

 the ventral side, the internal half being on the anterior border. The 

 articular surfaces on carpale 2, 3, 4, and 5, which meet their respec- 

 tive metacarpals are broadly concave from front to back. 



Digits.— ThQ metacarpal of the first finger is very smaU and the 

 finger is very short. The metacarpals gradually lengthen toward the 

 outer side of the foot, reaching their maximum development in the 

 fourth, which, as in Varanosaurus, is the longest toe of the foot. The 

 fifth toe is divergent, as clearly indicated by the articular surface of 

 carpale 5. 



The digital formula remains to" be definitely determined, though 

 there is every reason to beUeve it to be 2, 3, 4, 5, 3, as here illustrated 

 and as it exists in most primitive reptilian forms. 



' Case, E. C. Pelycosauria of North America, Carnegie Institution, Pub. No. 55, 1907, pp. H3, 114, pi. 

 16,flg. 1. 

 « Willlston, 8. W. American Permian Vertebrates, 1911, pi. 7, fig. 8. 



