Page:The Osteology of the Reptiles.pdf/213

Rh

The most primitive hand or manus known is that of the Cotylosauria, from the Permocarboniferous (Figs. 128, 133). The five metacarpals 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 remembered, however, that we know nothing whatever of the hands or feet of the earliest amphibians, and it is purely an assumption that the reptilian hands and feet were evolved from forms like the later ones of Permocarboniferous times. In all probability the embolomerous ancestors of the reptiles had the phalangeal formulae of both front and hind feet like those of the known earliest 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 ) as well as most modern lizards (Fig. 140 ) having the same number of bones arranged in the same ways. This primitive phalangeal formula is that of the Cotylosauria, Therocephalia, Theromorpha, Phytosauria, Pseudosuchia, Rhynchocephalia, Nothosauria, or at least some members of the group, and the group called by the author the Acrosauria, that is, the early Araeoscelis (Fig. 155 ), Protorosaurus (Fig. 138 ), Pleurosaurus (Fig. 139 ), and Sauranodon (Fig. 139 ). In the Crocodilia (Figs. 140, 157 ), the postaxial fingers are in all cases shorter and weaker, with fewer phalanges.