The bony framework, or skeleton, that which gives form and stature to the body, and which serves for the support of the soft parts and the attachment of muscles, is, with rare exceptions, all that is ever preserved of fossil animals. Because, therefore, students of extinct animals must rely so much, if not exclusively, upon the skeleton much attention has been given to the study of comparative osteology, the science of bones. Not only are most of the bones of the skeleton characteristic of the genus to which they belong, but the more general plan of the skeleton, or parts of the skeleton, is likewise characteristic of the larger groups. The paleontologist may become so expert in deciphering the characters of single bones, or even parts of bones—often all that are known of animals new to science—that he is able to hazard guesses as to the general structure of the skeleton to which they belong. But such guesses usually will approximate the real truth only in the degree that the bones upon which they are based approximate like bones of other animals that are better known. Not all parts of the skeleton are equally characteristic of the type of animal which possessed them. A tooth of a mammal may positively determine the species to which it belongs, while the toe bone of the same animal might not enable one to guess at its family, even. As a rule one can seldom be quite sure of the species of a reptile unless the larger part of the skeleton, or at least the skull, is available, although almost any bone of the skeleton, if one is expert, will permit a decision as to the family, if not the genus. One must often depend upon the positions and relations of the bones, as found in the rocky matrix, for the final determination of many characters. One can, for instance, never be sure of the number of bones in the neck, trunk, tail, or feet of a reptile, until specimens have been found with all such bones in position. It is for this reason that much care is exercised in the collection of specimens of fossil animals, and especially of fossil reptiles, to preserve all parts of the skeleton, so far as possible, in the relations they occupied in the rocks until they can be studied in the laboratory. Many grievous errors have been made in the past by hasty inferences from fragmentary and poorly collected specimens. [Illustration: FIG. 3.—_Limnoscelis_, a subaquatic cotylosaur, from the Permocarboniferous of New Mexico.] Because of the reliance which must be placed upon the skeleton it will be necessary to speak somewhat in detail of its structure in the reptiles, and to use not a few terms in its description that are unfamiliar to the general reader. So far as possible technical terms will be avoided, though some must be used, as there are no equivalents in the English language for them. The reader may use this chapter as a sort of explanatory index or glossary for the better elucidation of the necessary details of the following chapters.