Page:The Osteology of the Reptiles.pdf/217

Rh above the normal number (hyperphalangy) and of the digits (hyperdactyly) is known only in swimming animals. In some if not all Proganosauria (Fig. 153 ) the fifth toe has two extra phalanges, that is, 2, 3, 4, 5, 6, possibly but very improbably a primitive character, as the earliest foot known (Eosauravus, Fig. 151 ) from the middle Pennsylvanian has the same number and arrangement of the phalanges as in the Cotylosauria (Figs. 128, 133) and modern lizards (Fig. 140). In Trionyx, a river turtle, five phalanges have been observed in the fourth toe, and as many as six in the fifth, certainly an acquired character, and the only examples of hyperphalangy in the order Chelonia. In web-footed swimming animals there is sometimes a tendency toward the elongation of the fifth toe, as observed in Eosauravus (Fig. 151 ), Lariosaurus (Fig. 149), and especially Mesosaurus (Fig. 153 ), and Tylosaurus (Fig. 148). It may perhaps indicate the use of the hind legs more as sculling organs after the manner of seals, sea otters, and the Cretaceous bird Hesperornis, in all of which the fifth toe is very long and strong, though without additional phalanges.

In all strictly aquatic reptiles (Figs. 158, 159) the digits are elongated, and except in the Chelonia, there was an increase of the number of phalanges in both front and hind feet, sometimes far beyond the normal number. A like hyperphalangy is observed in the marine mammals, one or two additional cartilaginous phalanges in the sirenians, and from two to ten ossified ones in the Cetacea. Various theories have been proposed to account for their origin. That they cannot be due to the ossification and separation of the normal epiphyses in reptiles is quite evident, for these reptiles at least had no epiphyses. Like the additional epipodials of the plesiosaurs and ichthyosaurs, they are accessory, new ossifications in the mesenchyme and not reversions to a primitive fish-like fin.

In the Mosasauria there was a progressive increase in hyperphalangy as observed in the genera Clidastes (Fig. 146), Platecarpus (Fig. 147), and Tylosaurus (Fig. 148) from one or two to as many as six or eight additional phalanges, concomitant with the progressive chondrification of the mesopodials. In certain plesiosaurs as many as twenty-two phalanges are known in the third digit, and certain ichthyosaurs have even more. Hyperdactyly, due to the same causes, is known in the ichthyosaurs only among reptiles (Fig. 158 ).