OPHTHALMOSAUETJS. 9 



edge of the bone. Probably there was no actual contact either with the prootic or 

 opisthotic, broad tracts of cartilage having intervened between the several otic elements. 

 The upper border of the supraoccipital is gently convex from side to side and is also 

 curved forwards laterally ; the edge is broad and occupied by a deep roughened groove 

 (pa.f.), indicating that probably there was a pad of cartilage between this bone and 

 the overlying parietal. 



Tlie iwootic (text-fig. 3, A, B) is a very small bone, oval in outline, with the outer 

 surface gently convex in all directions, the convexity being most marked at one end 

 of the long axis. Its inner face bears a triradiate smooth channel corresponding to 

 the anterior vertical and horizontal semicircular canals ; round these is a roughened 

 border for cartilage, varying in width, the widest being at the most convex end of the 

 long axis. The precise position of this bone cannot be made out, as it does not appear 

 to have been in actual contact with either of the other otic elements, but was 

 surrounded by cartilage. Probably its position has been most nearly determined by 

 Bauer * in his figures given in his paper on the otic bones of this genus. 



The opisthotic (PI. I. fig. 15 ; text-fig. 3, E, F) is a large and solidly constructed bone, 

 the position of which with regard to the surrounding elements is not easy to determine. 

 The following account is founded on an examination of several sets of bones of the 

 occipital region, and may be taken as representing fairly exactly the actual condition 

 of things. It will be seen that the position here ascribed to the opisthotic approaches 

 most nearly to that described by Gilmore and shown in his figure of the skull of 

 Bajjtanodon f , but differs in the relations with the exoccipitals ; on the other hand, 

 the descriptions given by Bauer in Ophthalmosaurus %, and by E. Fraas §, Cope ||, and 

 OAven ^ in Ichthyosaurus, differ considerably from the present account. The bone 

 consists of an inner greatly thickened region and an outer short stout process directed 

 upwards and outwards, and terminating in a convex facet [sq.f.), which fits closely into 

 a corresponding surface on the inner face of the squamosal, in the angle between the 

 process of that bone which joins the parietals and the downwardly-directed portion 

 Avhich embraces the upper end of the quadrate (see text-figs. 4 & 8, A, B). This 

 outwardly directed bar of the bone bears both on its anterior and still more 

 markedly on its posterior face strong ridges and roughened tuberosities {t.) for union 

 with muscles or tendons. 



The expanded inner end of the bone is roughly trihedral ; ventrally it bears a 

 pair of rather narrow, roughly triangular surfaces {st.f. & st.f .) (the posterior of 



* Anat. Anzeig. vol. sviii. (1900) pp. 586-7, figs. 17, 18. 



t Mem. Carnegie Museum, vol. ii. (1905) p. 85, pi. xi. fig. 2. 



X Anat. Anzeiger, vol. xviii. (1900) p. 581. 



§ ' Die Ichthyosaurier der Siiddeutschen Trias- und Jura-Ablagerungen,' pi. ii. fig. 3. 



II Proc. Amer. Assoc, vol. xix. (1870) p. 199, fig. 2. 



1[ ' Foss. Eept. Lias. Form.' pt. iii. (Mon. Pal. Soc. 1881) pi. sxvi. fig. 1. 



