Many birds and fishes have a peculiar ossification of the usually tendinous outer covering of the eyeball, called the sclerotic membrane. These ossifications form a flattened or somewhat projecting 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 simple process that it is in mammals, where it is controlled by simple ciliary muscles which compress the lens, causing 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 eyeball. 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 commonly present in aquatic reptiles. Among terrestrial reptiles there are not a few examples of the ossification of such sclerotic plates, notably among the skink lizards. Every known form of extinct reptiles of aquatic habit had them, and even some of the subaquatic dinosaurs, like _Diplodocus_ and _Trachodon_. One may say with assurance that it is impossible for any reptile 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 we 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 disappeared for the most part from the skin of all truly aquatic reptiles and mammals.