THE LIMBS 199 



above the normal number (hyperphalangy) and of the digits (hyper- 

 dactyly) is known only in swimming animals. In some if not all 

 Proganosauria (Fig. 153 a) the fifth toe has two extra phalanges, 

 that is, 2, 3, 4, 5, 6, possibly but very improbably a primitive char- 

 acter, as the earliest foot known {Eosauravus, Fig. 151 b) 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 ob- 

 served 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 b), Lariosaurus (Fig. 149), and especially 

 Mesosaurus (Fig. 153 a), 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 elon- 

 gated, and except in the Chelonia, there was an increase of the num- 

 ber 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 epi- 

 physes 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 hyper- 

 phalangy 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 c, d). 



