114 



REVISION OF THE PELYCOSAURIA. 



Fig. 



radiale is a stout bone with a wide proximal face; the distal end is divided into two 

 articular faces which lie at an angle to each other. The ithiare is a thin disc-like bone, 

 much larger in size than the radiale. Between the two lies the intermedium, which 

 projects well beyond the proximal ends of the radiale and ulnare. Distal to the inter- 

 medium and ulnare lie two cenirale ; the distal ends of these lie almost on a line with 

 the distal end of the radiale. On the ulnar side of the cenirale 2 is 2. facet which, in 

 the specimen No. 1003, has nothing to articulate with it, but in specimen No. looi, 



where the foot bones are all present, 

 \ there is a thin disk-like sesamoid 



which occupied this position. The 

 distal row consists of five bones. 

 The articulation of the different 

 bones is plain from fig. i, plate 16. 

 The ulnare has on the radial side 

 two articular facets which meet 

 facets on the intermedium and 

 centrale 2 ; the union of these four 

 faces leaves a foramen. The first 

 carpal projects prominently from 

 ^-Vv V>VX ^>sj>^ /^ 17 n '^^^^ ^^^^ °^ ^^^ carpus and presents 



>J- x\. >Oyv Os^ Or r? 5 n a peculiar twisted surface which 



permitted a considerable range of 

 movement. 



The phalanges are not pre- 

 serv'ed beyond the proximal half 

 A little more can be made out 

 from one or two other specimens, 

 No. 114 and No. lOoi University 

 of Chicago. The first digit had the largest of the metacarpals ; the second digit was 

 perhaps the longest. From data derived from incomplete specimens in the American 

 Museum the phalangeal formula seems pretty certainly to be the usual 2, 3, 4, 5, 4 of 

 the Rhynchocephalia. 



The phalanges are relatively short, well-fonned and with distinct articular sur- 

 faces. They indicate, as does the carpus, a strong, flexible foot, such as would be 

 possessed by a carnivorous, terrestrial animal. The terminal phalanges show that they 

 were cased in strong claws. 



The posterior limb (No. i University of Chicago): They^>««r is strong and heavy, 

 with well-developed articular surfaces. The proximal end is rounded anteriorly and 

 deeply excavated posteriorly so that it is crescentic in section. There is no distinct 

 head, the whole proximal end being rounded and thickened. There is a strong rugosity 

 near the outer part of the convex posterior surface. The shaft is roundly quadrate in 

 section. The distal end is divided by a deep groove into two strong condyles, both 

 bearing articular faces. The inner condyle is the shorter and extends inwards at a 

 small angle to the shaft ; its articular face is almost entirely on the posterior surface, 

 and looks more backwards than downwards. The outer condyle is longer than the 

 inner and is directed downwards. It bears two articular faces; one on the posterior 

 surface, looking almost directly backwards, is nearly square in outline; the other is 

 apparently for the head of the fibula ; it is largely on the lower surface of the tuber- 

 osity and joins the posterior face by a narrow neck near its external side. 



radiate ; 

 , pisiform ; 



4) a — Restoration of anterior foot of Dimetrodon. r, 

 , intermedium; ;(, ulnare; fi and d, centrale; /, 

 /, 2, 3- 4< 5- carpals. 

 Fig. 41 i.— Restoration of posterior foot of Dimetrodon. f, fibulare ; 

 /, tibiale ; c, centrale ; ;', intermedium ; /, 2, 3, 4, J, tarsals. 



