282 VERTEBRATE SKELETON 



as having been made by birds. The reduction of the appendages 

 reaches its extreme in apodal Hzards and snakes, in which most, or 

 even all traces of hmbs are lost. 



The ancestral reptiles undoubtedly were terrestrial, but some 

 (some Chelonia, Ichthyosaurs, Pythonomorphs, Sauropterygia and 

 Thalattosauria) returned to the water, while the Pterosauria took 

 to the air. With these changes in habitat the limbs of the aquatic 

 forms became paddles, the fore limbs of the Pterosaurs were trans- 

 formed into wings. As in other cases of modification the stages 

 of these changes may be reconstructed in the aquatic groups. 



A webbing of the digits, as in marine turtles, prevents inde- 

 pendent motion of parts; then increase in length and breadth of the 

 limb made it a more effective paddle, increase in length being either 

 by elongation of the phalanges (Chelonia), or by increase in their 

 number (fig. 306) there being as many as twenty phalanges in some 

 digits of Ichthyosaurs and Sauropterygians; and in some the paddle 

 was widened by increase in the number of digits, either by division 

 or by additions on either side. The efficiency of the muscles was 

 increased by reduction of the proximal bones, the extreme being 

 reached on some Ichthyosaurs where stylopodial and zeugopodial 

 bones are polygonal and scarcely longer than the phalanges. The 

 modification of the fore hmbs of Pterodactyls for flight consist in 

 the elongation of the bones of the forearm and the fifth digit to 

 support the membranous, bat-Uke wing, the other digits, except the 

 vestigial first, being more normal. 



Theromorpha. — The limbs are less known than are the skulls. Most 

 members of the group had an entepicondylar foramen (large in Therapsids) and 

 some had one on the radial side. Therapsids also have a marked major tuberos- 

 ity. The long ulna of Cotylosaurs has an olecranon. The hind hmbs are much 

 like the anterior, pentadactyly prevaiUng. Some had a single centrale, some 

 more, and often all five tarsaha are distinct. 



Sauropterygia have Hmbs which are not quite complete paddles, and fore 

 limbs sometimes larger than the hind. The humerus is long and slender in the 

 more primitive genera; in others it is short and stout, a radial epicondylar 

 foramen occasionally occurring. Usually there are two proximal and five distal 

 ossicles in the carpus, with a pisiforme and traces of a postminimus. Five 

 metacarpals are present and the more primitive genera had the phalanges, 

 numbering 2, 3, 4, 4, 3, in the digits, beginning with digit one, the number being 

 increased to nine in some digits of later species. The pelvic Hmbs resemble the 

 anterior pair except in greater reduction of the tarsal bones. 



