22 



WATER REPTILES OF THE PAST AND PRESENT 



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 croco- 

 diles, 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 milHons 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, exoccipi- 

 tals, and supraoccipital, are almost invariably free and there is a 

 single occipital condyle, except in the Theriodontia. 



In this reduction or fusion of parts, or in addition thereto, there 

 has been a general lightening-up of the whole skull-structure in 

 reptiles from the rather massive and protected form of the older 

 to the lighter, less protected, and more fragile type of the 

 later ones, since speed, greater agilit}-. better sense organs, and 



Fig. 6. — Labidosaunis, a colylosaur. 

 Skull from above: pm, premaxilla; it, 

 nasal; m, maxilla; /, lacrimal; p, pre- 

 frontal; /r, frontal; />/, postfrontal; 

 po, postorbital; j, jup;al; pa, parietal; 

 sq, squamosal; ds, dcrmosupraoccipi- 

 tal; pj, parietal foramen. 



