OPHTHALMOSAURUS. 31 



vomers in some Ichthyosaurian skulls, and may be regarded with some confidence as 

 being those elements. 



Sclerotic Ring (PL I. figs. 9, 10). — The large and strongly ossified sclerotic ring in 

 the Ichthyosaurian eye is one of the most notable characteristics of the group, but 

 nevertheless does not seem to have been satisfactorily described. In the present 

 collection there are several more or less nearly complete specimens of this structure, some 

 with a greater or less number of sclerotic plates still united, others in which they 

 have all become separated. From these it appears that the plates not only formed a 

 ring round the pupil on the front of the eye, but externally also curved rather sharply 

 round on to the back of the eyeball, over which they extended some distance, though 

 not so far as in front. Owen* has already described this in the case of some Liassic 

 Ichthyosaui's, and points out that the structure of the ling indicates " the extreme 

 oblateness of that visual spheroid." Each separate plate extends from front to back, of 

 course narrowing towards the pupil ; in the young the plates are very thin, but they 

 become considerably thickened with advancing age. They unite along their edges in a 

 complex suture (PI. I. fig. 10), interlocking in such a way that no movement can have 

 taken place between them. Certainly in no specimen have they been seen to overlap 

 simply one another as has been figured by Gilmore in the case of Baptanodonf. 



Even in birds, especially the Hawks and Owls, in which the plates often imbricate, 

 they frequently fit into grooves in one another's edge, and in any case are so firmlv 

 united by connective tissue that little or no movement between the separate elements 

 is possible. As already noticed, the plates are very thin in the young; they seem to 

 ossify from a centre lying just in front of the point where they bend round to the 

 back of the eye, and at this centre of growth there is a rugose surface from which 

 there radiates in all directions a series of fine lines, which give the bone a fibrous 

 appearance. Growth in thickness takes place by the addition of successive laminse, 

 probably on the inner side only ; the edges of these laminae of two contiguous plates 

 interlock in the manner above described. 



Mandible (figs. 20-23). — The structiu-e of the mandible, on the whole, agrees very 

 closely with that described by Gilmore in the case of the American forms. Each 

 ramus consists of six elements — one, the articular, being a cartilage-bone, the re- 

 mainder sheathing membrane-bones. Of these latter the dentary [dent.) is the largest: 

 anteriorly it forms the whole of the jaw and terminates in a point ; posteriorly it runs 

 back, overlapping the anterior ends of angular and surangular on the outer face of 

 the ramus, and terminating in a point a little in front of the coronoid process of the 

 surangular refeiTed to below ; internally it extends back about the same distance as 

 externally, but in the specimens examined its posterior end is concealed by the over- 

 lapping coronoid bone. The outer face is convex from above downwards, and a little 



* ' Kept. Lias. Form.' (Mon. Pal. Soc. ISSl) pt. iii. p. 103. 



t '■ Notes on Osteology of Baptaaodon," Mem. Garuegie Museum, vol. ii. (1906) p. 328, fig. 3. 



