on the Fossil Genera Ichthyosaurus and Plesiosaurus. Ill 



to have been a projecting- process, broken otf from the inner side of the coro- 

 noid bone : 3^ is a portion of the articular bone of the lower jaw. 



In the left lower jaw of the head figured in PI. XVII, the interior is well dis- 

 played ; and this projecting process from the inner side of the coronoid seems 

 distinct. In this specimen, however, the crescent-shaped bone cannot be 

 clearly made out, the jaw being slightly crushed and fractured in this part. 



I wish to direct the further inquiries of those who may possess specimens, 

 to these points. Dry and uninteresting as such details must necessarily be, 

 they acquire importance from their connexion with so essential a point as the 

 muscular action employed in moving the lower jaw. 



Although in the points which have been mentioned, the angular and coro- 

 noid bones (or rather the latter) of the ichthyosaurus approximate more nearly 

 to those of other lacertae than of the crocodile ; yet I have next to notice a 

 configuration, very striking in the character it imparts to the posterior part of 

 the jaw, which brings them back again to the latter type ; namely, their extend- 

 ing, as in the crocodile, so far back, as to cover and conceal on the exterior 

 the whole of the articular bone ; whereas in the other lacertoe the articular 

 bone, forming the posterior end of the lower jaw, is displayed for some distance 

 on the outside as well as the inside. 



5. Tlie Articular bone. — This affords room for no remark, except in being 

 concealed on the outside, both in the crocodile and ichthyosaurus, as just stated. 

 It may be seen, marked 3/, in the sketch of the inner side of the jaw, fig. 14. 

 PI. XVI. ; and in the left lower jaw of the head, PI. XVII. 



6. The Opercular hone, — affords no distinctive characters*, but very closely 

 ao;rees with that of the crocodile. 



On the whole, then, if the lower jaw in the ichthyosaurus is compared with 

 that of the crocodile and other lacertae, the analogies will stand as follows : — 

 The extension of the angular and coronoid bones, so as to cover the articular, 

 renders the posterior extremity exactly like that of the crocodile ; and the fur- 

 row carrying the teeth, though strictly speaking sui generis, yet approximates 

 most nearly to the alveolar dental of the crocodile. On the other hand, the per- 

 forations for the exterior distribution of the branches of the lower maxillary 

 nerve, closely resemble those in the lacertae ; and the absence of the oval hole 

 is a character of the same kind, though common perhaps to some of the fossil 

 species of crocodile also. Tlie structure of the coronoid process appears to 

 hold an intermediate place. 



* This bone is very little developed in some of the lacertx ; but in otliers it acquires almost as 

 great an extent as in (be crocodile. 



