252 MEMO I as OF THE QUEENSLAND MUSEUM. 



The opisthotic or paroccipital very rarely appears to meet the basi- 

 occipital below the exoccipitals in other reptiles, although the relations between 

 these elements are variable. The writer has diffidence in using terms that are 

 not accepted by leading authorities, but the occipital region of the Ichthyosaurs 

 provides material for several interpretations, and the use of exoccipital for the 

 lower element seems to solve the difficulty of a most anomalous " stapes." 



In the modern cetaceans the stapes is frequently reduced to a small conical 

 plug, and, judging from analogy, the auditory functions- of the Ichthyosaurs would 

 not have been greatly utihsed. Possibly the real stapes is the " long slender 

 process '" demonstrated in Section 494 of the very fine series in Sollas' great work 

 {loc. cit.). 



The stapes is often missing in fossils. The elaborate studies by D. M. S. 

 Watson of the position of the fenestra ovalis in Therapsids, Seymouria, etc. 

 (P.Z.S., 1914, and 1919) have an important bearing here, but the posterior 

 aspect of the occipitalia in our specimen presents no evidence on this point. 



The massive architecture of the occipital region was evidently associated 

 with the attachment of powerful nuchal muscles. Perhaps a specialist will one 

 day work out details of the probable musculature of the Ichthyosaurs on similar 

 lines to the recent studies by Gregory and Camp on Cynognathus.'^^ 



Lower Jaw. — On the left hand side the lower jaw is practically complete,, 

 except for the missing anterior segment and a small portion of the angulare. The 

 dentary is very elongated and is no less than 875 mm. in maximum length. Parallel 

 with the alveolar border, and situated about 20 mm. below it, is a groove which 

 is shalloAV anteriorly, but then deepens, giving the characteristic conjoined gun- 

 barrel effect of the Ichthyosaurian rostrum. The posterior process of the dentary, 

 which overlaps the surangulare, runs back to below the mid-region of the supra- 

 temporal fossa. Here the semi-spherical contours (in section) of the upper rod, or 

 gun-barrel-like process, sink into the same plane as the surangulare and angulare. 



The left angulare is not quite complete at its posterior end, and here its 

 outer contours have been disturbed. It is a longer and more massive bone than the 

 surangulare, but just at the termination of the dentary the two posterior elements 

 are of equal depth. In this region the arrangement of the bones is very similar to 

 the outer view given by C. W. Andrews for Ophlhalmosmirvs in Text-fig. 20 {loc. cit.), 

 except that the angulare is distinctly extended to form the posterior portion of the 

 mandible. Strong depressor muscles weie evidently attached here, working with 

 short leverage in association with the powerful levators placed in front of the 

 articulation. Sollas points out that the levator muscles originating in the temporal 

 region, inserted on the lower jaw, acting as levers of the third order, were "admirably 

 adapted for snapping ; and the Ichthyosaurus, from all that we know of it, must have 

 obtained its food by seizing fish ' upon the wing.' " 



20 Gregory and Camp, Bull. Amer. Mus. Nat. Hist. XXXVIII, 1918, pp. 447-563. 



