Class 6. Mammalia. 



473 



sacrum; the false ribs, shorter. The sternum, which is almost 



always long and narrow, consists at first of a cartilaginous mass, in 



which a series of ossifications appears later; the latter usually remain 



separate throughout life, so that the adult sternum retains a jointed 



appearance ; occasionally they fuse to a great 



extent (as in Man) , The most anterior joint, 



the manubrium, is generally somewhat 



broader than the succeeding ones ; to this 



the first pair of ribs are attached, whilst the 



other true ribs are attached at the junctions 



of the other joints. The last joint, the 



xiphisternum {processus xiphoides) , with which 



no ribs are connected, usually ends in a broad 



cartilaginous plate. Only in the Monotremes 



is there an episternum corresponding 



with that of Reptiles, as in many Lizards it 



is here a large T-shaped bone. 



The skull of the adult consists chiefly 

 of bone, and exhibits but little cartilage. Not 

 only the small premaxilla and the large 

 maxilla, but also the bones belonging to 

 the upper' portion of the gill-bars, are fused 

 with the skull. Of these only the palatine, 

 which is attached anteriorly to the premaxilla, 

 and the somewhat small pterygoid are 

 present, whilst the quadrate has dis- 

 appeared (or at least in its usual form, see 

 below under the ear) ; the lower jaw, which 

 consists of a single bone on each side, 

 articulates directly with the skull. The two 



rami are either united anteriorly by means of connective tissue, 

 or (in the adult) are anchylosed (Horse, Man, and others). 

 There are two occipital condyles instead of one as in 

 Reptiles and Birds. There is no interorbital septum as in many Reptiles, 

 etc.; the cranial cavity extends forwards as far as the nasal 

 cavities. The latter are usually very well developed; they are 

 separated by a plate, at first cartilaginous, later partially replaced by 

 bone, which arises from the anterior wall of the skull, and projects 

 forwards. They are also at first surrounded laterally and dorsally by 

 outgrowths from the anterior region of the cartilaginous cranium, but 

 after a time these ossify, or are covered in by membrane bones, and 

 then dwindle away; those portions which surround the external nares 

 and the adjacent regions persist, however, throughout life. In the adult 

 the nares are surrounded by various bones, laterally chiefly by 

 the maxillffi, dorsally by the well-developed, flattened nasals, 

 which touch in the median line; ventrally by the palate 



Fig. 384. Sternum and 

 clavicle of a Kangaroo. cZ 

 clavicle, 7n manubrium, i 

 ribs (out oil) , x xiphisternum, 

 x^ its cartilaginous terminal 

 plate. — Orig. 



