BRITISH FOSSIL REPTILES. 8/ 



companied by an important modification of the articular surfaces 

 of the vertebral centres, each of which surfaces presents a well- 

 marked concavity, leading to the inference that they were ori- 

 ginally connected together by an elastic capsule filled with a 

 fluid, as in the vertebral joints of the back-bone of Fishes, and 

 the Perennibranchiate or most fish-like of the Reptilia. 



The structure of the fins of many species oi Ichthyosaurus de- 

 viates from that of the Cetacean paddles, and approaches in cer- 

 tain peculiarities more closely to that of the fins of Fishes, than 

 has yet been found in any other reptile. First, the digits exceed 

 the typical number _five, and resemble in their numerous and 

 small constituent phalanges the jointed rays which support the 

 natatory membrane of the pectoral and ventral fins of true 

 Fishes ; and, secondly, numerous cartilaginous bifurcate rays 

 were added to the bony apparatus which supports the tegumen- 

 tary expansion. 



With these important modifications of the head, trunk, and 

 extremities in immediate relation to aquatic progression, the law 

 of the correlations of organic structure would lead us to antici- 

 pate some corresponding modification of the tail. Accordingly 

 we find the vertebrae of this part to be much more numerous 

 than in the previously-described Enaliosaurian group. There 

 is no trace, however, of any confluence of the terminal caudal 

 vertebrae, or of any modification of their elongated neur- and 

 hffim-apophysial spines, such as form the characteristic struc- 

 ture supporting the tail of the osseous Fishes. The numerous 

 caudal vertebrae gradually decrease in size to the end of the tail, 

 where they assume a compressed form ; and thus the tail, in- 

 stead of being short and broad as in Fishes, is lengthened out 

 as in the Crocodiles. 



The very frequent occurrence of a fracture of the tail about 

 one fourth of the way from its distal extremity, had led me to 

 suspect it to have been connected with the presence of a tegu- 

 mentary caudal fin ; and the laterally compressed form of the 

 terminal vertebrae, since ascertained by Sir Philip Grey Egerton, 

 gives additional demonstration both of the existence and direc- 

 tion of such a fin. The only evidence, in fact, which the skele- 

 ton of the Cetaceous mammal aff'ords of the powerful horizontal 

 caudal fin which characterizes the recent animal is the depressed 

 or horizontally flattened form of the terminal vertebrae. We 

 may infer, therefore, from the corresponding vertebrae of the 

 Ichthyosaurus being flattened in the vertical direction, or from 

 side to side, that it possessed a caudal tegumentary fin expanded 

 in the vertical direction ; and it would be highly advisable to 

 examine narrowly the lias matrix in which the tail of the Ich- 



