BRITISH FOSSIL REPTILES. 97 



coronoid is more developed than in the Crocodiles, but ap- 

 pertains, as in them, to the surangular element. The jaw of 

 the recent Crocodilians is characterized bj^ a large vacancy 

 which occurs between the angular, surangular, and dentarj-^ 

 pieces, while the surangular itself is imperforate. In the La- 

 certians no vacancy occurs between the above-named maxillary 

 elements, but in some genera, as the Iguana, the surangular is 

 perforated. Now, in this highly characteristic deviation from the 

 Crocodilian structure, we find the Ichthyosaurus participating 

 with the inferior or Lacertian Saurians ; and in some species, as 

 the Ich. communis, the surangular bone exhibits a well-marked 

 perforation. Tt is interesting to observe that in the Teleosaurus 

 and Steneosaurus the vacuity between the angular and surangu- 

 lar pieces is reduced to a very small size, which, in combination 

 with the modification of their vertebral column is evidence of 

 their transitional character between the great carnivorous Sau- 

 rians of the past and present epochs. In the backward exten- 

 sion of the surangular bone the Ichthyosaurus manifests an affi- 

 nity, not exclusively to the Crocodiles, but to certain Lizards 

 also, as the large Monitors, in which there is a corresponding 

 development of the surangular bone in that direction. In the 

 depth of this piece, on the other hand, the Ichthyosaurus stri- 

 kingly resembles the Lacertian and deviates from the Crocodilian 

 type. The conformation of the posterior angle, the robustness 

 of the articular extremity of the lower jaw, in short, all those 

 characters that relate to the muscular forces destined to wield an 

 instrument armed with numerous and large destructive teeth, ap- 

 proximate the Ichthyosaurian jaw more nearly to the Crocodiles 

 than to the feeble Lizards ; but in those characters which more 

 truly indicate affinities to a typical structure, as being less liable 

 to modification for particular functions, the Ichthyosaurus de- 

 cidedly manifests its closer affinity to the Lacertian character. 



The intermediate or annectent characters of the Ichthyosau- 

 rus between the Crocodiles and Lizards is exemplified in a re- 

 markable degree in the modification of that part of both the 

 upper and lower jaws which is destined for the support of the 

 teeth. 



The Plesiosaurus, like the Crocodile, has its teeth lodged in 

 distinct sockets : the Lacertian Sauria have their teeth anchy- 

 losed, like the teeth of most fishes, to the alveolar process of 

 the jaws, which process is a simple plate corresponding to the 

 outer wall of the alveoli in the higher Reptiles 3 the inner alveolar 

 plate being very slightly, if at all, developed. In the Ichthyo- 

 saurus both the outer and inner plates of the alveolar groove are 

 present, and the teeth have their bases free, as in the Crocodiles, 



1839. H 



