LIMBS 



165 



most Snakes. In certain of the latter, howevei, traces of the 

 hind limbs exist (e.g. Python). 



The tibia gradually becomes of relatively greater size than the 

 fibula in the reptilian series. The tarsus always undergoes a 

 marked reduction, especially in its proximal portion, and gradually 

 leads to the type seen in Birds. Thus in Chelonians and Lizards 

 (Figs. 129 and 130) the proximal tarsals may all run together into 

 a single mass, which in the former corresponds to the tibiale, 

 intermedium, fibulare, and centrale. In Lizards a centrale can 

 no longer be recognised, even in the embryo, and there is no distinct 

 trace of an intermedium. In the distal row three or four separate 

 tarsals are developed, but these may unite with one another 

 to a greater or less extent, and there is an increasing tendency 



77 



Ufa 



Fiu. 130. RIGHT TARSUS OF 

 From above. 



F, fibula; T, tibia; t.f.i.c, fused 

 tibiale, intermedium, fibulare, and 

 centrale ; f, trace of a 6th ray 

 present in Geckos ; 35, distal 

 tarsals ; I V, metatarsals. 



FIG. 131. RIGHT TARSUS OF CROCO- 

 DILE. From above. 



Fj fibula ; /, fibulare (calcaneum) ; 

 T, tibia ; t,i,c, astragalus, corre- 

 sponding to fused tibiale, inter- 

 medium, and centrale ; 1, 2, 3. 

 fused 1st 3rd distal tarsals ; 4, 4th 

 distal tarsal ; / IV, metatarsals ; 

 F?, 5th tarsal and metatarsal. 



for the movement of the foot to take place by means of an inter- 

 tarsal articulation, as in the Dinosauria and also in Birds. 



In Crocodiles (Fig. 131) there are two bones in the proximal 

 row of the tarsus, one of which corresponds to a tibiale, inter- 

 medium, and centrale, the other to a fibulare. The former is 

 spoken of as the astragalus, the latter as the calcaneum, and on it 

 a definite heel (calraneal process) is seen for the first time in the 

 animal series. The distal row consists originally of four 'small 

 cartilages, but these later undergo a partial fusion. 



Birds. In consequence of the fore limb of Birds having 

 become adapted for flight, the maims loses its primitive character 

 and undergoes reduction, while the humerus and the bones 

 of the fore arm more particularly the ulna, as well as the 

 entire pectoral arch and sternum, are extraordinarily developed, 



