THE LIMBS 195 



Metapodials and Phalanges 



The most primitive hand or manus known is that of the Cotylo- 

 sauria, from the Permocarboniferous (Figs. 128, 133). The five meta- 

 carpals increase in length to the fourth; the fifth is shorter, but is not 

 markedly divaricated. There are two phalanges in the thumb or 

 pollex, three in the second digit, four in the third, five in the fourth, 

 and but three in the fifth. The first and fifth metacarpals are more 

 freely movable on the wrist than are the other three. 



Of the Temnospondylous amphibians no complete hand is known. 

 That there were five functional digits is certain,^ since there are five 

 functional carpalia in both Eryops (Fig. 136) and Trematops. It is 

 often assumed that all amphibians of the past, as of the present, had 

 but four fingers, as is known to be the case in some of the ancient 

 Stegocephalia. The phalangeal formula was either 2, 3, 4, 4, 3 or 2, 

 3, 3, 4, 3, in the rhachitomous temnospondyls. It must be remem- 

 bered, however, that we know nothing whatever of the hands or feet 

 of the earliest amphibians, and it is purely an assumption that the 

 reptihan hands and feet were evolved from forms like the later ones 

 of Permocarboniferous times. In all probabiUty the embolomerous 

 ancestors of the reptiles had the phalangeal formulae of both front 

 and hind feet like those of the known earhest reptiles. We can hardly 

 conceive of an increase either of the number of digits or number of 

 phalanges in the earliest reptiles. 



In crawling reptiles the structure of the digits, it is seen, has not 

 changed much to the present time, the modern Sphenodon (Fig. 

 139 A, b) as well as most modern lizards (Fig. 140 c, d) having the 

 same number of bones arranged in the same ways. This primitive 

 phalangeal formula is that of the Cotylosauria, Therocephalia, The- 

 romorpha, Phytosauria, Pseudosuchia, Rhynchocephalia, Notho- 

 sauria, or at least some members of the group, and the group called 

 by the author the Acrosauria, that is, the early Araeoscelis (Fig. 

 155 a), Protorosaurus (Fig. 1380), Pleurosaurus (Fig. 139 d), and 

 Sauranodon (Fig. 139 c). In the Crocodilia (Figs. 140 a, 157 a, b), 

 the postaxial fingers are in all cases shorter and weaker, with fewer 

 phalanges. 



1 [For a different interpretation, however, see Gregory, Miner, and Noble in 

 Bulletin, Amer. Mus. Nat. Hist., 1923, vol. xlviii. — Ed.] 



