538 MAMMALS. 



quadrate, and columella or hyo-mandibular of other Verte- 

 brates. The otic bones fuse to form a compact periotic. 

 A bony palate, formed from premaxillx, maxillae, and pala- 

 tines, separates the buccal cavity from that of the nasal 

 passages. In most cases there are teeth, borne in sockets 

 by the premaxillse, maxillae, and mandible. 



Except in Monotremes, the coracoid is represented merely 

 by a small process from the scapula, forming part of the 

 glenoid cavity in which the head of the humerus works, but 

 not reaching the sternum. The latter is divisible into three 

 regions ; — (a) a presternum with which the interclavicle is 

 fused, and with which the clavicles (if well-developed) 

 articulate, {b) a.mesosternum divided into segments, with 

 which the sternal parts of the ribs articulate, and (c) a 

 xiphisternum, often cartilaginous. There are generally two 

 sacral vertebrae, with which several caudals, and more rarely 

 a lumbar, may be fused. The ilia slope downwards and 

 backwards, the ischia have no symphysis, but the pubes are 

 almost always united ventrally. 



Nervous System. — The cerebral hemispheres have usually 

 a convoluted surface, and always cover over the optic thalami 

 and the optic lobes (now four-fold corpora quadrigemina), 

 and in higher forms the cerebellum as well. The commis- 

 sural system is well-developed, being especially represented 

 by a large corpus callosum, except in Monotremes and 

 Marsupials, in which the anterior commissure is large and 

 the corpus callosum incomplete. There is also an import- 

 ant set of longitudinal fibres called the fornix. 



Alimentary System. — Except in the Monotremes, in which 

 there is a cloaca, the food-canal ends separately from the 

 urinogenital aperture. 



Circulatory System. — The heart is four-chambered, and 

 the temperature of ihe blood is high, though less than that 

 of Birds. There is but one aortic trunk, which curves over 

 the left bronchus. The red blood-corpuscles are, when 

 fully formed, non-nucleated, and are circular in outline, 

 except in the Camelidse where they are oval. 



Respiratory System. — The lungs are invested by pleural 

 sacs, and lie freely in the cavity which is bounded posteriorly 

 by the diaphragm. Within the lungs the bronchial tubes 

 fork repeatedly into finer and finer branches. At the top 



