ADAPTATION OF LAND REPTILES TO LIFE /.V WATER 71 



jecting conical bony ring about the pupil of the eye. The individual 

 bones are flat and more or less imbricated plates, with some motion 

 between them. Accommodation for vision in reptiles, birds, and 

 fishes is not the simjile process that it is in mammals, where it is 

 controlled by simple ciliary muscles which compress the lens, caus- 

 ing it to assume a more spherical or a more flattened form, thus 

 changing the focus. In reptiles accommodation is effected by the 

 compression of the eyeball by means of external muscles, elongating 

 it and causing its front part to expand or project. The imbricated 

 sclerotic plates permit this expansion and contraction of the eye- 

 ball. Under great internal or external air pressure the cornea, the 

 only unprotected part, must necessarily change its contour unless 

 some compensatory force is brought to bear to counterbalance it; 

 and this doubtless was the function of the sclerotic plates so com- 

 monly present in aquatic reptiles. 



Among terrestrial reptiles there are not a few examples of the 

 ossification of such sclerotic plates, notably among the skink hzards. 

 Every known form of extinct reptiles of aquatic habit had them, and 

 even some of the subaquatic dinosaurs, like Diplodocus and Tracho- 

 don. One may say with assurance that it is impossible for any rep- 

 tile to become thoroughly adapted to aquatic life without acquiring 

 large and strong sclerotic plates. 



Most land reptiles are or were covered by horny scales or bony 

 plates; the pterodactyls are the only order of terrestrial reptiles 

 with no such covering of which w-e have any evidence. Such 

 coverings are wholly unneeded for animals living in the water. 

 Not only are they unnecessary, but the increased resistance to the 

 water would be more or less detrimental to rapid swimming. It 

 is for these reasons doubtless that bony plates or horny scales dis- 

 appeared for the most part from the skin of all truly aquatic reptiles 

 and mammals. 



The foregoing are the chief acquired characteristics of aquatic 

 air-breathing animals and especially aquatic reptiles in adaptation 

 to their new mode of life. The resemblances, sometimes striking, 

 thus brought about in animals of very different origin and remote 

 relationships have often been mistaken for evidences of kinship, 

 that is, direct inheritance from common ancestors. Such acquired 



