GENERAL CHARACTERS OF MAMMALS. 695 



quadrate which intervenes in Sauropsida has disappeared, or 

 has been shunted to become one of the ear ossicles. For it is 

 a plausible theory of the three ossicles malleus, incus, and 

 stapes which connect the drum with the inner ear, that they 

 correspond respectively to the articular, quadrate, and columella 

 or hyo-mandibular of other Vertebrates^- The otic bones fuse 

 with each other to form a compact periotic. A bony palate, 

 formed from premaxillcz, maxillce, and palatines, separates 

 the buccal cavity from the nasal passages. In most cases there 

 are teeth, borne in sockets by the premaxillce, maxillce, and 

 mandible. 



Except in Monotremes, the coracoid is represented by a 

 small process from the scapula, and sometimes by a small 

 ossification, forming part of the glenoid cavity in which the 

 head of the humerus works. The sternum includes (a) a 

 prcesternum, with which the clavicles (if well developed} 

 articulate ; (b) a mesosternum divided into segments, with 

 which the sternal parts of the rib? articulate ; and (c) a 

 xiphisternum, often cartilaginous. There are generally two 

 sacral vertebrce, but several caudals, and more rarely a 

 lumbar, may be fused to these. The ilio-sacral articulation 

 is in front of the acetabiilum. The ventral symphysis is 

 usually restricted to the pubes, but in some Insectivores and 

 Bats these do not meet. Except in Echidna, the acetabulum 

 is completely ossified, and there is often a special acelabular 

 bone, the ankle joint is cruro-tarsal. 



The cerebral hemispheres have usually a convoluted surface, 

 and always cover the optic thalami and the optic lobes (now 

 fourfold corpora quadrigemind), and in higher forms the 

 cerebellum as well. The commissural system is well developed, 

 being especially represented by a large corpus callosum, except 

 in Monotremes and Marsupials, in which the anterior com- 

 missure is large and the corpus callosum absent or very small. 

 There is also an important set of longitudinal fibres called the 

 fornix. 



Except in Monotremes, in which there is a cloaca, the food 

 canal ends separately from the urogenital aperture. 



The heart is four-chambered, and the temperature of the 

 blood is high, though lower than that of Birds. There is but 



1 There are many other theories as to the quadrate, e.g. that it form? 

 the malleus. 



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