118 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
are interpreted as rudimentary additional digits—prehallux, prepol- 
lex, postminimus—but their statusisuncertain. Therearealsocertain 
membrane bones developed in the appendages including the patella 
(knee-pan) in some reptiles, birds and many mammals, in the tendon 
that passes overghe knee-joint, the fabellz in the angle of the knee of 
a few mammals, ‘and the pisiforme 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. 
nie a Besides there is frequently a torsion of the bones 
ae Affi of the forearm (fig. 127) or shank on each other. 
®. wil “a ‘g The lower amphibians have nearly typical legs, 
L although, as in Siren and Amphiuma, they may be 
Ss, <\ greatly reduced, while in some stegocephals and 
Fic. 126. Fic. 127. 
Fic. 126.—Tarsus of Geotriton, after Wiedersheim, showing the arrangement of the 
metapodial elements. c¢, centrale; f, fibulare; F, fibula, 7, intermedium; #, tibiale; T 
tibia; 1-5, tarsales. . 
Fic. 127.—Torsion in developing human arm, after Braus. u, 7, 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- 
