As in other parts of the skeleton, there has been a reduction in the number of parts of the reptile skull from that of the more primitive forms, and a better adaptation of those which remain for the special uses they subserve. This reduction in number has been caused in part by the actual loss of bones, in part by the fusion of contiguous ones. The most primitive reptiles had no less than seventy-two separate bones in the skull;[1] the human skull has but twenty-eight inclusive of the ear bones. There is but little variation, either in the number or in the relations of bones, in the mammalian skull. If one knows the human skull thoroughly he can easily understand the structure of the skull of any mammal. The same cannot be said of the skulls of reptiles; one would be greatly puzzled in the comparison of the skulls of turtles and crocodiles, if he knew nothing about other forms. And it is safe to formulate another general law in evolution here: Characters which have been longest inherited are least liable to change. The earliest reptiles had at least four pairs of bones which have disappeared in all later reptiles; and they had some bones in pairs which have fused in later reptiles, either with their mates or with contiguous bones. The crocodile has at least two pairs of bones which have disappeared in turtles. On the other hand, the turtle has at least one pair of free bones which have been fused with adjacent bones in the crocodiles, and one pair that is fused which is free in the latter. The lizard has one pair of bones that has been wholly wanting in other reptiles for millions of years, while on the other hand it has lost some bones that are present in all other modern reptiles. The four parts of the occipital bone of mammals, basioccipital, exoccipitals, and supraoccipital, are almost invariably free and there is a single occipital condyle, except in the Theriodontia. [1] Paired maxillae, premaxillae, nasals, prefrontals, lacrimals, frontals, parietals, dermosupraoccipitals, tabularia, supratemporals, intertemporals, squamosals, jugals, quadratojugals, postorbitals, postfrontals, quadrates, exoccipitals, paroccipitals, vomers, palatines, pterygoids, sphenomaxillae, stapes, transverse, alisphenoids or orbitosphenoids, epipterygoids, articulars, prearticulars, angulars, surangulars, coronoids, splenials, dentaries, one supraoccipital, one basioccipital, one basisphenoid, one ethmoid. [Illustration: FIG. 6.—_Labidosaurus_, a cotylosaur. Skull from above: _pm_, premaxilla; _n_, nasal; _m_, maxilla; _l_, lacrimal; _p_, prefrontal; _fr_, frontal; _pf_, postfrontal; _po_, postorbital; _j_, jugal; _pa_, parietal; _sq_, squamosal; _ds_, dermosupraoccipital; _pf_, parietal foramen.]