n8 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



are interpreted as rudimentary additional digits prehallux, prepol- 

 lex, postminimus but their status is uncertain. There are also certain 

 membrane bones developed in the appendages including the patella 

 (knee-pan) in some reptiles, birds and many mammals, in the tendon 

 that passes over the knee-joint, the fabellae in the angle of the knee of 

 a few mammals, and the pisif orme in the carpus of man and some other 

 mammals. 



In the ancestral limb, as exemplified in the urodeles, the basal 

 joint was directed horizontally at right angles to the axis of the body, but 

 higher in the scale it approaches the sagittal plane and in such a way 

 that the angles of the fore and hind limbs open in opposite directions. 

 Besides there is frequently a torsion of the bones 

 of the forearm (fig. 127) or shank on each other. 

 The lower amphibians have nearly typical legs, 

 although, as inSiren and Amphiuma, they may be 

 <^> \\\\ // // greatly reduced, while in some stegocephals and 



^ 



FIG. 126. FIG. 127. 



FIG. 126. 'Tarsus of Geolriton,. after Wiedersheim, showing the arrangement of the 

 metapodial elements, c, centrale; /, fibulare; F, fibula, i, intermedium; t y tibiale; T 

 tibia; 1-5, tarsales. 



FIG. 127. Torsion in developing human arm, after Braus. , r, ulna and radius; 

 dotted line, course of radial nerve. 



the gymnophiones they are entirely lacking. In the anura the radius 

 and ulna or tibia and fibula are frequently fused and the tarsals 

 elongated. 



The most marked feature of the reptilian limb is the occurrence of 

 an intratarsal joint, the motion of the foot upon the leg being largely 

 between the two rows of tarsal bones, instead of between tarsus and the 

 bones of the shank (fig. 128). There is also a greater range of form 

 than in the amphibia. Limbs are lacking in snakes and some lizards; 

 on the other hand there is a great increase in the number of phalanges, 

 correlated with a shortening of the proximal bones in the plesiosaurs, 

 which reaches its extreme in the ichthyosaurs where there may be a 

 hundred phalanges in a digit. The wings of the pterodactyls are re- 



