560 CLASS XVII. 



animals, on the contrary, the posterior, or sometimes all the ribs 

 are attached to the transverse processes alone of the dorsal vertebrae. 

 The parts of the ribs which are united to the sternum remain in 

 most of the mammals cartilaginous during the whole life; in some 

 only are these pieces, as in birds (see above, p. 330) early ossified, 

 as in Bradypus, Dasypus, Myrmecophaga, the Cetacea, the Mono- 

 tremata. The anterior ribs always extend as far as the sternum, 

 and are thus true ribs, in which respect again the mammals differ 

 from the birds (see above, p. 330). In the mammals the number 

 of. true ribs (of which there are mostly seven, eight or nine) com- 

 monly exceeds that of the false, that is, of those which are not 

 connected with the sternum. 



The sternum in mammals consists mostly of many bony pieces 

 lying behind each other in a row ; their form sometimes corresponds 

 with that of the caudal vertebras, but also is often flat. In most 

 mammals this bone is narrow and lengthened, in the cetacea it is 

 broad. A projecting ridge, as in the sternum of birds, occurs only 

 in some burrowing mammals (as in the mole) and in the bats as 

 exceptions. 



The anterior limbs are never absent in mammals. They are 

 not always provided with a clavicle and by it connected with the 

 sternum. The cetacea, namely, the ungulata and some of the 

 edentata, have no clavicle. An imperfect clavicle, that does not 

 extend from the top of the scapula to the sternum, is present in 

 some rodents and in the greater number of carnivorous animals, 

 amongst which it appears to be wanting in some few only. The 

 insectivorous mammals, many rodents, all quadrumanous animals 

 and bats, have constantly a perfect clavicle. Usually this bone is 

 long and then not unlike a rib; in the mole it is very short and 

 more thick than long, and provided in front with a process ; it has 

 a large articular surface for the humerus, and is on the contrary 

 united to the top (acromion) of the shoulder by ligament only. In 

 the genus Bradypus the clavicle is not united to the acromion but 

 to the coracoid process of the scapula. The clavicle of mammals 

 is the os furculare of birds (see above, p. 330). A second or cora- 

 coid clavicle is found in the monotremes only (the Ornitliorhynchus 

 and the Echidna}. 



The scapula is always present. It is a flat, triangular or elon- 

 gated bone, of which the inner surface, which lies upon the ribs, is 



