THE ICHTHTOSATJEIA. . 245 



elevations, or articular surfaces, which are at first situated in 

 the upper haK of the lateral surface. Towards the posterior 

 half of the dorsal region they descend, and, gradually- 

 approaching one another, coalesce into one in the caudal 

 vertebree. The form of the proximal ends of the ribs cor- 

 responds with the arrangement of these tiibercles; for, 

 whei-e they are separate, the proximal end of the rib is 

 forked. The lower fork, or capitiikmi, goes to the capitular, 

 or lower, tubercle, and the upper branch, or tuberculum, to 

 the upper, or tubercular, elevation. In the caudal region, 

 where the articular surface is single, the proximal end of 

 the rib is also undivided. In the caudal region the ribs are 

 short and straight, but in the precaudal region they are 

 stout and curved, and much longer in the middle than at 

 either end of the series. The atlas and axis resemble the 

 other vertebrae in their general form : but a wedge-shaped 

 bone is, as it were, let in between their opposed lower edges ; 

 and a similar bone, attached to the under-part of the con- 

 cave face of the centrum of the atlas, serves to complete the 

 cup for the occipital condyle. 



The skull of Ichtlmjosaurus (Fig. 76, A) is remarkable for 

 the great elongation and tapering form of the snout, the 

 huge orbits, the great supra-temporal fossae, and the closing 

 over of the infra-temporal fossae by plates of bone. Again, 

 the two rami of the mandible are united in a symjDhysis, 

 which, for length, is comparable to that observed in the 

 modern Gavials and in the ancient Teleosmiria. The basi- 

 occipital bone furnishes the rounded articular condyle to 

 the first vertebra, and l^ecomes very stout and thick in front. 

 It appears to have been ankylosed neither with the basi- 

 sphenoid nor with the basi-occipital. The latter bones are 

 adapted to its sides, and, together with the supra-occipital, 

 which is interposed between them above, circumscribe the 

 occipital foramen. The basisphenoid, a deep and stout 

 bone, is produced in front into a long and slender para- 

 sphenoidal rostrum. There do not appear to have been 

 any ossified alisphenoids. The parietals remain separate 

 throughout life ; and, in some species, not merely present 



