90 REPORT — 1839. 



near its middle a vertical depression, as if for the insertion of 

 some ligament. 



I may here observe that in true Fishes the concave articular 

 surface is present on the body of the posterior cranial verte- 

 bra, or occiput, as it is on the bodies of the ordinary vertebrae. 

 The deviation from this character in the Ichthyosaurus, and the 

 substitution of a diametrically contrary structure, bespeaks 

 strongly its true Saurian nature. In the Cetacea the basi-occi- 

 pital forms no part of the articulation with the vertebral column, 

 but the head is joined to the atlas by two ex-occipital condyles 

 as in other Mammalia. 



The ex-occipital s are proportionally smaller in the Ichthyo- 

 sauri than in the Crocodiles, and do not unite together so as 

 to complete the boundary of the foramen magnum above, but 

 allow the supra-occipital element to form about one third of the 

 upper circumference of this foramen. This approximation to the 

 Lacertian type, of which the discerning eye of Mr. Conybeare* 

 had led him to entertain a suspicion from mutilated specimens, 

 I have ascertained beyond doubt to be a generic structure in the 

 Ichthyosauri. 



Two very strong mastoid bones extend from the ex-occipitals 

 towards the articular extremity of the tympanic bone, and nearly 

 obliterate the space intervening at the back part of the skull 

 between the parietal bifurcations and the occipital bone. 



The solid structure of the back part of the cranium which thus 

 ensues gives to the skull of the Ichthyosaurus a strong resem- 

 blance to that of the Crocodile ; but as this is an adaptive rather 

 than a typical conformation, it affords but a slender argument 

 for their affinity. The development of the occipital bones in 

 both cases depends on the necessity for a due extent of surface 

 for the implantation of the powerful nuchal muscles which must 

 have mainly wielded a head produced anteriorly into long and 

 heavy jaws beset with numerous and formidable teeth. 



The upper part of the cranium includes the parietals, the 

 composite frontals, and the principal elements of the temporal 

 bones. 



The parietals form together a strong triradiate bone, as in the 

 Plesiosaurs. The temporal muscles, which derive part of their 

 origin from its median and anterior portion, extend to the middle 

 line, where they are separated by an osseous intermuscular crest. 

 Anterior to this crest, close to or in the coronal suture, the pa- 

 rietal is perforated, — a structure not present in the Crocodiles, 

 but peculiarly characteristic of the Lacertian Saurians, and the 



* Geol. Trans., 1822, p. 117. 



