GENERAL CHARACTERS OF MAMMALS. 695 
guadrate which intervenes in Sauropsida has disappeared, or 
has been shunted to become one of the ear ossicles. For it ts 
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, guadrate, and columella 
or hyo-mandibular of other Vertebrates The otic bones fuse 
with each other to form a compact pertotic. A bony palate, 
Jormed from premaxille, maxilla, and. palatines, separates 
the buccal cavity from the nasal passages. In most cases there 
are teeth, borne in sockets by the premaxille, maxilla, and 
mandible. 
Except in Monotremes, the coracoid ts 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 
presternum, 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 vertebra, but several caudals, and more rarely a 
lumbar, may be fused to these. The ilio-sacral articulation 
is in front of the acetabulum. The ventral symphysis ts 
usually vestricted to the pubes, but.in some Insectivores and 
Bats these do not meet. xcept in Echidna, the acetabulum 
is completely ossified, and there is often a special acetabular 
bone. The ankle joint ts cruro-tarsal. 
The cerebral hemispheres have usually a convoluted surface, 
and always cover the optic thalamit and the optic lobes (now 
jourfold corpora quadrigemina), and in higher forms the 
cerebellum aswell. 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 ts large and the corpus callosum absent or very small. 
There 1s also an important set of longitudinal fibres called the 
Jjornix. 
Except in Monotremes, in which there is a cloaca, the food 
canal ends separately from the urogenital aperture. 
The heart ts 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 forms 
the malleus, 
