on the Fossil Genera Ichthyosaurus and Plesiosaurus. 1 13 



are horizontally disposed : thus the strength of the part is greatly increased by 

 a regular diagonal bracing, without the least addition of weight or bulk. A 

 similar structure may be noticed in the overlapping bones of the heads offish, 

 and in a less degree also in those of the turtle. 



IV. Nostrils and Anterior Part of Upper Jaw. — Some general re- 

 marks may be added to what has been said in the former memoir on the struc- 

 ture of these parts. 



Nature designed both the ichthyosaurus and the crocodile to possess elon- 

 gated muzzles ; but she has in the two instances varied the usual Saurian type^ 

 in order to produce this effect in a very different manner. In the crocodile 

 she has given extraordinary length to the maxillary bones ; and, removing the 

 nostril from its usual place, transferred it to the extremity of the intermaxillary 

 bones. The reason for this may probably be, that the crocodile lurks for prey 

 near the banks of rivers, with the tip alone of his long snout out of the water. 

 At that point, therefore, the nostril was necessarily placed, to enable him to 

 scent his food. But to the ichthyosaurus, living in the sea, such a position 

 of the organ would have been useless : as in the lizard, therefore, in the 

 monitor, and indeed in most quadrupeds, this opening is placed between the 

 nasal, maxillary, and intermaxillary bones ; and the prolongation of the snout 

 is principally effected by an unusual elongation of the intermaxillary bones. 



The position of the nostril in the crocodile, moreover, gives unusual deve- 

 lopment to the nerves of the olfactory organ ; the sense of smell must 

 therefore be very acute in that animal. But in the ichthyosaurus, the parts 

 connected with the organ being small in their proportions, the sense was pro- 

 bably dull. This has been insisted on as an important distinction between the 

 two genera *. 



V. Eye and Orbit. — These parts give rise to no new remarks. The bony 

 plates of the sclerotica present a difference from the crocodile, and an agree- 

 ment with the other lacertae, 



VI. The Temporal FosSiE. — In the head of the crocodile, two fossas may 



* It should be observed, that In the sketches of the head of ichthyosaurus accompanying the 

 first memoir, the posterior end of the intermaxillary bones has been carried somewhat too far 

 back, being made completely io encircle the opening of the nostril ; whereas it should have been 

 confined to the anterior margin of that opening, which is bounded by it ; as is the lower margin 

 by the lacrymal, and the superior by the nasal bone. The position of the nostril between these 

 three bones answers to that in the lacertas. This part is correctly represented in PI. XVII. of 

 the present volume, 



VOL. VI. <2 



