on the Fossil Genera Ichlhj/osaitrus and Plesiosaurus. 105 



First ; in comparing the large lioles behind the orbit in the ichthyosaurus with 

 those in the crocodile^ I ought to have noticed that they have a still closer ana- 

 logy with the temporal fossae in the other lacertae. The use of the cavities in 

 question in the skull of the crocodile not having been, at that time, clearly ex- 

 plained, and these cavities having been erroneously described as peculiar to 

 that animal, an obscurity was thrown over the whole of this part of the subject. 

 Subsequent information and my own further examination have satisfied me, 

 that they are the true temporal fossae of the crocodile, and do not differ, except 

 in their smaller size, from those of the remaining lacertae. The ichthyo- 

 saurus, in respect to this part, appears to hold an intermediate place between 

 the crocodile and the lacertae ; but certainly approximates most nearly to the 

 latter : so that thus far I must correct my former statements. 



The second point to which I am desirous of adverting, in hmine, is the 

 representation given of the roof of the mouth, in fig. 12. PI. 40, Vol. V. I 

 have reason to believe, from the examination of nearly perfect specimens since 

 discovered (see PI. XVII. of the present volume), that the fractured state of the 

 specimen from which the drawing for the former plate was taken, and also the 

 irregular line of section it presented, have occasioned an exhibition of these 

 parts in some respects incorrect. 



The figures of the head of the ichthyosaurus, given in the former communi- 

 cation, were, in many instances, restorations made up from the comparison 

 of many different specimens. Indeed, had not this method been adopted, it 

 would have been impossible to communicate, without a large apparatus of 

 engravings, the general information which alone it was my object, at that time, 

 to impart. In my present communication I shall submit drawings from indi- 

 vidual specimens only ; a method far more satisfactory, where it is practicable 

 within reasonable limits : and it will be seen that these figures fully confirm 

 the restorations attempted in the preceding memoir. 



In proceeding to the details which I now propose to offer, I shall con- 

 sider, 



I. The Dentition of the Ichthyosaurus. — In my former paper on this 

 subject, following the authority of Sir Everard Home, I simply stated a gene- 

 ral agreement with the dentition of the crocodile : but from the importance 

 attached to this branch of the subject by most anatomical writers, I have since 

 been induced to examine the matter more minutely ; especially as I have found 

 a disposition in some scientific friends, to whom my observations were commu- 

 nicated, to compare the dentition of the ichthyosaurus rather with that of the 

 other lacerta*. It will be necessary, in the first instance, to state in what points 



VOL. VI. p 



