OPHTHALMOSArRTJS. 1 1 



from it by a notch, is a pointed process, bearing in some cases a small facet {exo.f.) for 

 articulation with the outer upper facet of the exoccipital ; in cases where the facet 

 was wanting, no doubt the actual junction of the two bones was prevented by 

 the intervention of a pad of cartilage. The junction of the opisthotic with the 

 exoccipital above and the basioccipital below, encloses between it and the former of 

 these two bones an oval vacuity, through which must have run some of the nerves and 

 vessels passing into the jugular foramen which, as above described, notches the front 

 of the exoccipital. The upper and anterior face of the inner end of the opisthotic 

 formed the posterior, outer and lower portion of the auditory capsule, and is deeply 

 impressed by two grooves meeting towards the inner end and marking the position of 

 part of the posterior vertical (jj.v.c.) and of the horizontal (h.c.) semicircular canals. 

 Round these the bone forms a roughened border of varying width for cartilage, and 

 probably it had no contact with either the supraoccipital or epiotic elements. The 

 chief peculiarities of this bone (e. g., its extensive union with the basioccipital and 

 stapes) all seem to tend to increasing the rigidity of the squamoso-quadrate 

 complex. 



The stapes (PL I. fig. 12 ; text-figs. 3, C, D, & 4), as in other Ichthyosauria, has under- 

 gone a most peculiar modification. Instead of being, as in most Reptiles, a slender rod 

 of bone connecting the tympanic drum with the inner ear, it seems to have lost its 

 auditory function and has become a stout bar of bone, lying between the basioccipital and 

 quadrate and helping to add to the rigidity of the latter. The proximal end is greatly 

 enlarged and forms a massive head, the inner face of which is occupied by a surface 

 {hoc.f.), slightly convex from above downwards, by which it articulates with the 

 corresponding facet on the basioccipital. Above this surface on the upper side of 

 the head are two obliquely elongated facets {op.f. & op.f .) separated by a groove; 

 these, as above described, articulate with the corresponding surfaces of the opisthotic, 

 the groove forming the lower half of the channel [g.) running between the two 

 elements. The posterior face of the head is flattened and a little roughened ; the 

 anterior face is produced ventrally into a blunt triangular prominence, from which a 

 ridge (r.) runs obliquely upwards on to the narrow outer process of the bone. This 

 latter terminates in a blunt point, which bears a facet, looking upwards and forwards, 

 for union with the corresponding pit on the inner face of the quadrate. In addition to 

 its inner and outer unions with the basioccipital and quadrate respectively, the lower 

 angle of this bone is wedged into the angle formed by a ventral shelf-like process of 

 the portion of the pterygoid which is united to the inner face of the quadrate (see 

 pterygoid below, also text-fig. 4). This union added still further to the rigidity of the 

 occiput. 



The arrangement of the various bones described above is figured in text-fig. 4. 

 In this the form of the foramen magnum is well shown, as also is the remarkable 

 manner in which the quadi'ate is supported by the stapes, opisthotic (indirectly), 

 squamosal, and pterygoid. 



c2 



