MERRTAM: TRTASSTC IOTTTIIYOSATTRTA. 10f> 



about 50 per cent, larger than in specimen 9950 of C. petrinus, in which the 

 length was over thirty feet. The anterior portion of the rostrum of speci- 

 men 995-1 is absent, but the remaining part of the skull is over five feet in length. 



Skull. The structure of the skull in this type is fortunately almost per- 

 fectly shown in specimens 9950 and 9913 from American Canon (pis. 2-6). 

 Other skulls, some of which are nearly as complete, show similar structure. 

 In no. 9950 the skull is preserved intact, and has been almost entirely freed 

 from the matrix with relatively little difficulty. Excepting a slight depression 

 of the frontal region practically no distortion has occurred. 



Though it is relatively primitive in many respects, the skull in this form is 

 typically ichthyosaurian. Like the typical Jurassic genera, the facial region 

 is relatively long and slender, the premaxillaries and nasals are very largely 

 developed, the frontals are small or are largely covered by the nasals, the 

 superior nares occupy a nearly median position, and a heavy sclerotic ring is 

 present. In contrast to the later forms the maxillaries are relatively larger, 

 the snout shorter, the orbits smaller, and the lateral temporal region broader 

 aiiteroposterioiiy. In these respects the skull is more primitive than any de- 

 scribed form from a later period. 



Compared with the length of the trunk the skull is relatively short. As 

 nearly as can be estimated, the length equals a little more than one-third that 

 of the dorsal region. The skull of Mixosaurus seems to be relatively much 

 longer, equaling about two-thirds the length of the trunk, while in Ichthyo- 

 saurus the relative length may be as great as 80 per cent. 



In this species the antorbital portion of the skull is elongated as a rostrum, 

 which narrows gradually along practically the whole distance out toward the an- 

 terior end, though the diminution in width is less noticeable immediately beyond 

 the anterior ends of the maxillaries. The rostral region represents about 60 per 

 cent, of the total length of the skull, but is relatively shorter than in any other 

 ichthyosaurian with which the writer has any acquaintance, excepting Ichthyo- 

 saurus breviceps, from the Lower Lias, in which the length is 55 per cent. The 

 extent of the maxillary anterior to the orbit is relatively larger than in any other 

 ichthyosaurian. It amounts to 48 per cent, of the length of the antorbital region 

 in this form, while in /. breviceps with a somewhat shorter rostrum, in which one 

 would expect to find the maxillary relatively long, it is less than 40 per cent. 



The premaxillaries extend more than half the total length of the skull. For 

 a little more than half of their length behind the anterior end of the rostrum 

 they meet dorsally. Behind this point they are separated by the nasals. In 

 specimen 9950 the posterior ends pass above the anterior ends of the superior 

 narial openings, but do not appear to extend any distance below them as seems 

 to be the case in most other ichthyosaurians. In another specimen (no. 9954) 

 the superior narial opening is cut into the posterior part of the premaxillary 



