312 PBOF. IX. G. SEELET ON SIMILITUDES OE 



§ 10. The Piscine Characters of Ichthyosaurus. 



In the general form of the head, Xifhias resembles Ichthyo- 

 saurus. The premaxillaries in both are elongated, terminate the 

 long snout, and form the alveolar border, which in Xiphias does 

 not carry teeth ; they are followed behind in both by a pair of 

 greatly elongated triangular nasals, which widen behind. 



The frontals and parietals of Xiphias are both large ; the pre- 

 frontal is covered by nasal and frontal ; and the prefrontal and 

 postfrontal do not meet to exclude the frontal from the orbit. 

 Neither orbits nor temporal fossae are circumscribed ; and there is 

 no foramen parietale. 



The basioccipital is conically cupped, and carries the exocci- 

 pitals, which meet below the neural canal, and furnish concave 

 facets which contribute to the articulation with the vertebral 

 column, which is thus very vinlike that oi Ichthyosaurus. The ex- 

 occipitals also enclose the neural arch above. On the palate, too, 

 there is nothing in common, the bones being single and closing the 

 palate mesially. In very few fishes is there a convex basioccipi- 

 tal ; among such is Fistularia. 



The vertebral column of fishes, though as variable as the skull, 

 never approximates in any genus to that of Ichthyosaurus, except 

 in the biconcavity and sometimes in the shortness of the cen- 

 trum. The essential difference from a fish is in having the ribs 

 articulated to the centrum by two tubercles. 



The arches and limbs do not correspond at all. 



§ 11. The Plesiosaurian Characters of Ichthyosaurus. 

 In Plesiosaurus the snout is never so long as in Ichthyosau- 

 rus, or so pointed ; but the nostril is near the eye ; and nostril, 

 orbit, and temporal fossa are all surrounded with bone, though 

 relatively to Ichthyosaurus the temporal fossa is large and the orbit 

 small. The premaxillary is like that of Ichthyosaurus, only much 

 smaller ; and the nasals, which extend between the nostrils in 

 front and behind, are narrow and small relatively to those of Ich- 

 thyosaurus. The frontal appears in Plesiosaurus to enter into the 

 upper margin of the orbit ; and the postfrontal appears to meet 

 the malar and close the orbit behind ; so that there are no post- 

 orbital or supraquadrate bones. And the outer margin of the 

 temporal fossa is made by the malar meeting the squamosal ; 

 whether there is a quadrato-jugal I find no satisfactory evidence. 



