352 FOSSIL REPTILES. 



The /. plati/odon, in which this crown is compressed, and 

 presents on each side a trenchant ridge. The individuals of 

 this species vary in length from five to fifteen feet. 



The /. tenuirostris has the teeth more nairow, and the 

 muzzle longer and more slender. 



Lastly, the /. intermedins has the teeth more sharp, and 

 less profoundly striated than those of /. communis, less narrow 

 than in tenuirostris. 



The last two species do not attain to more than half the 

 size to which /. communis may arrive. 



As these species do not differ in the rest of their conforma- 

 tion but by slight variations in the proportions of the bones, 

 but not in the composition of the parts, we shall first give a 

 generic description of the head as if it belonged but to one 

 species, noticing the differences after. 



The elongated and pointed muzzle of the ichthyosaurus is 

 formed principally by two bones, furnished with teeth, and 

 which, in all its anterior moiety, are united to each other, 

 above and below, by a suture, but are separated in the upper 

 moiety by two other bones, which advance between them in a 

 point. ^ 



On each side of this same upper half is seen a narrow bone, 

 in which the series of the teeth is continued, and which is 

 prolonged behind as far as under the anterior angle of the 

 orbit. 



There are no nostrils at the point of the muzzle ; their aper- 

 tures are two oblong holes, in the summit of the intermaxil- 

 laries ; from their anterior edge they form an emargination in 

 the upper edge of the intermaxillaries. 



The upper and internal edge of the nostrils is made by the 

 nasal bone, which widens to arrive there, forming a notched 

 suture with the base of the intermaxillary. 



To the upper edge, that is, nearest the orbit, two bones con- 

 cur, or, at least, approach, which from their lower part also 

 concur to the formation of the circle of the orbit ; and even 



