GENERAL CHARACTERS OF THE MAMMALIA. 277 



or less imossified. In tlie Monotremes there is a T-shaped 

 bone above or in front of the prassternum, but this is to be 

 regarded as belonging to the shoulder-girdle, and as repre- 

 senting the " episternum " or " interclavicle " of the Eeptiles. 



The normal number of limbs in the Mammalia is four, 

 two anterior and two posterior ; and hence they are often 

 spoken of as "quadrupeds," though all the limbs are not 

 universally present, and other animals have four limbs as 

 well. The anterior limbs are not known to be wanting in 

 any Mammal, but the posterior limbs are absent in the 

 Cetacea and Sirenia. 



As regards the structure of the anterior limb, the chief 

 points to be noticed concern the means by which it is con- 

 nected with the trunk. The scapula or shoulder-blade is 

 never absent, and it is in the form of a broad flat bone, 

 applied to the outer aspect of the ribs, and much more de- 

 veloped than in the Birds. The coracoid bone, which forms 

 such a marked feature in the scapular arch of Aves, is fused 

 wdth the scapula, and only articulates with the sternum in 

 the Duck-mole and Echidna {Monotremata). In all other 

 Mammals the coracoid forms merely a process of the scapula, 

 and does not reach the top of the breast-bone. The collar- 

 bones or clavicles never unite in any Mammal to form a 

 " furculum," as in Birds ; but in the Monotremes they unite 

 with an " interclavicle " placed in front of the sternum. 

 The clavicles, in point of fact, are not present in a well- 

 developed form in any Mammals except in those which use 

 the anterior limbs in flight, in digging, or in prehension. The 

 Cetacea, the Hoofed Quadrupeds ( Ungulata), and some of the ' 

 Edentata, have no clavicles. Most of the Carnivora and some 

 Piodents possess a clavicle, but this is imperfect, and does 

 not articulate with the top of the sternum. The Insectivor- 

 ous Mammals, many of the Eodents, the Bats, and all the 

 Quadrumana, have (with man) a perfect clavicle articulating 

 with the anterior end of the sternum. 



The humerus, or long bone of the upper arm (hraclimm), 

 is never wanting, but is extremely short in the Whales, in 

 which the anterior limbs are converted into swimminR-- 

 paddles. 



