Chap. IX.] 



FORE LIMBS. 



353 



In Galeopithecus (the so-called Flying Lemur), the 

 fore and hind limbs are both elongated, and there 

 stretches between them, attached to the sides of the 

 body, a fold of skin which, unlike 

 the wing membrane of a bat, is hairy 

 on either side. Among lizards a 

 flying form is represented by Draco, 

 where the support for the flying 

 apparatus is afforded by the elonga- 

 tion of the ribs. 



Another set of modifications is 

 to be found in the aquatic penta- 

 dactyle forms, such as the turtles, 

 crocodiles, and aquatic mammalia. 

 Here the essential modification con- 

 sists in the elongation of the manus 

 or pes to form a fin- like organ ; the 

 simplest and first change is seen in 

 river tortoises, where, as in the feet 

 of wading birds, the digits are 

 merely connected by a web ; as this 

 web extends, it gradually encloses 

 the separate digits and converts the 

 organ into a paddle, as in the marine 

 turtles, where the fore are larger 

 than the hind limbs, or the whales 

 and Sirenia, in which the hind limbs 

 are altogether aborted ; the Sirenia 

 have rudimentary nails, and agree 

 with other mammals in never having 

 more than three phalanges to their 

 digits. The whales present a more 

 extreme case, as all rudiments of 

 nails are lost, and the phalanges of some of the digits 

 may come to be as many as twelve or thirteen in 

 number (Fig. 150). 



In the Mammalia we observe tha.t the limbs 

 x 16 



Fig. 149. Manus of 

 Bat. 



p, Pollex ; sc, scaphoid ; 

 wii to m*, the elon- 

 gated metacarpals of 

 the second to fifth 

 digits. 



