THE LIMBS 169 



The posterior epipodials in aquatic reptiles (Figs. 159, 158) are 

 almost indistinguishable from the anterior ones, except that they are 

 somewhat, or much, smaller. As in the front leg there may be acces- 

 sory epipodials in both the plesiosaurs and ichthyosaurs. This short- 

 ening of the epipodials, so characteristic of aquatic animals, is seen to 

 a moderate extent in the earhest known reptile, Eosauravus (Fig. 

 151 b) from the middle Pennsylvanian, as also in the Proganosauria 

 (Fig. 153 a) and Choristodera. It is much more pronounced in the 

 Mosasauria (Figs. 146-148, 158 a, b), Aigialosauria, Thalattosauria, 

 and Thalattosuchia (Fig. 150). The elongation of the tibia and 

 fibula, so characteristic of the cursorial or leaping forms, reached the 

 maximum in the Pterosauria (Fig. 155 c). 



The tibia and fibula of some temnospondylous amphibians are 

 quite indistinguishable from those of many Cotylosauria. 



Mesopodials 



Numerous modifications have occurred in the structure of the car- 

 pus and tarsus of reptiles in adaptation to diverse habits of life. The 

 carpus (Fig. 134) or wrist of the earhest known reptiles is composed 

 of eleven freely articulated bones, none small: four in the first row, 

 called respectively, from the preaxial to the postaxial side, the radiate, 

 intermedium, utnare, and pisiform, corresponding quite to the sca- 

 phoid, lunar, pyramidal, and pisiform of the human wrist; two in the 

 second row, the radial or first, and the ulnar or second, centrale; and 

 five in the third row, the carpaha, the first four corresponding to the 

 trapezium, trapezoid, magnum, and unciform of the human wrist. 

 Watson has recognized a small third centrale in the curious genus 

 Broomia (Fig. 137 e) from South Africa, unknown as an ossified ele- 

 ment in other reptiles, though perhaps represented by a cartilage in 

 the young of the modern Sphenodon. 



Carpus 



The carpus is known in but two temnospondylous amphibians, 

 Eryops (Fig. 136) and Trematops. In both, the preserved bones are 

 the same in number as in the early reptiles and some modern ones. 

 The radius of Eryops, however, articulates with three bones, the sup- 

 posed radiale, intermedium, and ulnare, while the pisiform is large, 

 and an articular surface on the postaxial distal margin of the ulna 



