106 COMPARATIVE ANATOMY. 



corresponds to the so-called pisiform bone of Mammals. The 

 distal row of carpals is much less developed than the proximal. 



In the fossil Flying Eeptiles (Pterodactylus, Rhauiphorhynchus) the fourth 

 finger was produced into a long jointed rod, which supported a wing-like 

 expansion of the integument. 



In all Reptiles, the tarsus undergoes considerable fusion, 

 especially in its proximal portion, and leads gradually on to the 

 type seen in Birds. Thus in Chelonians and Lizards the proximal 

 tarsals all run together into a single mass, which corresponds to 

 tibiale, intermedium, fibulare, and centrale. Traces of an extra 

 radial ray are also present here. 



In the distal row five separate tarsals are developed, but these 

 may unite partly with one another (Cheloniaus), and partly with the 

 corresponding metatarsals (Lizards), and thus there is an increas- 

 ing tendency for the movement of the foot to take place by means 

 of anintertarsal articulation, as in Birds. 



In Crocodiles, there are two bones in the proximal row of 

 the tarsus, one of which corresponds to a tibiale, intermedium, 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 

 (calcaneal 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 reduction. 



Birds. While the skeleton of the manus of Archasopteryx (Fig. 

 92) agrees in many points with that of Reptiles, that of existing 



FIG. 92. ANTERIOR EXTREMITY OF Archceopteryx. (After C. Vogt. ) 



Birds has become considerably modified by adaptation for flight, 

 and correspondingly reduced. Of the five carpals of the embryo, 

 the three distal become fused with the corresponding meta- 

 carpals (Fig. 93, Me, Me), while the two proximal remain separate 

 as a radiale and an ulnare. The metacarpals themselves become 

 in part united together, and only bear a very limited number of 

 phalanges at their free ends. 



The small size of the head of the humerus, as well as the absence of a 

 ridge for the insertion of the pectoralis major, and the probable small size of 

 the sternum, prove that Archseopteryx could not have been a good flier. In 



