152 VARIOUS FORMS OF LIMBS IN" MAMMALS. 



tribe's), certain digits are endowed with special offices, and 

 by a particular position enabled to oppose the others, so 

 as to seize, retain, and grasp. Lastly, in Man^ the offices 

 of support and locomotion are assigned to a single pair 

 of members ; the anterior, and now the upper, limbs being 

 left free to execute the various purposes of the will, and 

 terminated by a hand, which, in the matchless harmony 

 and adjustment of its organization, is made the suitable 

 instrument of a rational being. 



In contemplating and comparing the skeletons of a 

 series of mammals, the most striking modifications are 

 observable in the structure and proportions of the limbs. 



There are a few osteological characters in which all 

 mammalia agree, and by which they differ from the lower 

 vertebrata ; and some have been supposed to be peculiar 

 to them that are not so. The pair of occipital condyles, 

 e. g., developed from the exoccipitals, are a repetition of 

 what we saw in the batrachia. The flat surfaces of the 

 bodies of the trunk-vertebroe were a character of many 

 extinct reptiles ; but these surfaces in mammals are de- 

 veloped on separate epiphysial plates, which coalesce in 

 the course of growth with the rest of the centrum. Mov- 

 able ribs, projecting freely (pleurapophyses) in the cervical 

 region, may be found in a few exceptional cases (sloths, 

 monotremes); bony sternal ribs (h£emapophyses) exist in 

 most Edentata; a coracoid extending, as in birds and 

 lizards, from the scapula to the sternum, with an "epi- 

 coracoid," as in lizards, is present in the monotremes 

 (platypus or duck-mole, and echidna or spiny ant-eater, 

 of Australia) ; the cotyloid cavity may be perforated in 

 the same low mammals as in birds ; the digits may have 

 the phalanges in varying number in the same hand, and 

 exceeding three in the same finger, e. ^., in the whale 



