22 GEOLOGICAL SURVEY 
spinous processes small, and hence the spinal column moves, without obstruction, vertically, but 
with difficulty in a lateral direction. 
These motions are intimately connected with the wants and habits of these animals; but it would 
lead us beyond our purpose to consider them here—our object being simply to point out some of the 
principal characters derived from the solid parts of animals, that may give useful hints to the Geol- 
ogist in his investigations. 
The ribs may be regarded as vertebral elements, and the limbs and other bones of the 
skeletons of animals have been referred to modifications of similar elements, or as accessories to 
the spinal column. From the analogy found to exist between the mode of developement of 
vertebrae, and that of the cranial bones, comparative anatomists have been induced to consider the 
latter as highly modified forms of vertebrx ; and this analogy is strikingly constant, as appears from 
the fact that wherever the elements of the vertebrae remain unanchylosed, a like condition is 
observed in the elements of the cranial vertebree—facts that enable the Palzontologist to distinguish, 
at a glance, the remains of cold from those of warm blooded animals. 
The following are the orders of the class Mammalia. 
Class Mammalia. 
Quadrumana —Monkies. 
Cheiroptera —Bat, §rc. 
Carnivora —Bear, Cat, Wolf. 
Marsupiata = —Oposswm. 
Rodentia —Squirrel, Rat. 
Edentata —WSloth, Armadillo. 
Pachydermata—Hlephant, Hog. 
Solidungula —BHborse. 
Ruminantia —— Ox, Sheep. 
Cetacea — Whale, Porpoise. 
Characters of the Class—-In Mammalia the cranium articulates with the spinal column by 
means of a double condyle. 'The lower maxilla, which consist each of a single bone, articulate 
distinctly with the cranium, without the intervention of other bones. ‘The articulation varies with 
the habits of the amimal; in Carnivora the condyle fits into a hinge-like groove, which admits 
only of a vertical motion in the jaw. In the Herbivora provision is made for lateral motion; and in 
Rodentia a backward and forward motion of the jaw is indicated. The lower maxillary angle 
has a process, which, in Carnivora and Rodentia, is extended backwards. In Marsupiata, the angle 
is expanded laterally, terminating inwards in an acute process, as in the opossum, 
The application of microscopic research to the teeth of animals has given rise to a new science, 
called by Prof. Owen, Odontography, by which the affinities of animals may be determined by the 
microscopic characters of their teeth. 
In general teeth are composed of three elements, dentine, cement, and enamel. In herbivorous 
animals these are disposed in vertical plates, variously folded and convoluted, as may be seen in 
the tooth of a horse or ox. Being of unequal hardness, the grinding surface of the teeth is always 
uneven and rough—conditions necessary to the proper fulfilment of their functions, as instruments 
