THE NATURE OF LIMBS. 188 



tyles, camelus, inosclius, camelopardalis, cerviis, antelope, 

 ovis, and bos. 



As we "have now traced both tlie fore and hind foot to 

 the five-toed or pentadactyle structure, with the definite 

 number of joints or phalanges in each toe, characteristic 

 of the highest class of vertebrate animals, a few remarks 

 will be offered in illustration of the plan of structure 

 which prevails in such extremities, and of the law that 

 governs the departure from the pentadactyle type in the 

 mammalia. 



The essential nature of the limbs is best illustrated by 

 the fish called protopterus, and by some of the lower 

 reptiles that retain gills with lungs. 



If tlie segment of the skeleton supporting the rudi- 

 ments of the fore-limbs in the protopterus (Fig. 32), be 



PROTOPTERUS. 



compared with the modification of the typical vertebra, 

 exemplified in Fig. 6, p. 28, it will be seen to be con- 

 structed on the same type. The haemal arch is most ex- 

 panded, and it is composed of a pleurapophysis or verte- 

 bral rib, p^, and a hsemapophysis or sternal rib, /?, on 

 each side; the hsemal spine, or sternum, is not here de- 

 veloped ; the long, many -jointed ray, a, answers to the 

 more simple diverging appendage, r/, in Fig. 5. 



