©N GENERIC CHARACTERS IN THE ORDER SAUROPTERYGIA. 133 



9. On Generic Characters in the Order Sauropterygia. By 

 Prof. Owen, C.B., F.K.S., F.G.S., &e. (Bead December 20, 



1882.) 



The progress of knowledge of the species of Beptilia associated by 

 De la Beche under the collective name Enaliosanria, led to the sub- 

 division of that maritime group into the orders Ichthyopterygia and 

 Sauropterygia *, these terms being significative of their characters 

 of resemblance respectively to Fishes and to Saurians. 



Large accessions of species have since been made known in both 

 orders. With regard to the first, I have not deemed the modifica- 

 tions in the dentition, in the shape and structure of the sterno- 

 coraco-scapular frame, in those of the fins, or the gradations of 

 general bulk, sufficient, as satisfactory characters, for generic sub- 

 division. 



In the Sauropterygia, besides gradations of size, ranging, for ex- 

 ample, from Plesiosaurus Hawhinsii to Pies. Cramptoni, there is a 

 difference in the proportional length of the neck and number of its 

 vertebrae relating to the size of the head it supports. In Plesio- 

 saurus homalosjpondylus, for example, the cervical vertebrae are 

 thirty-eight in number; in Pies, rostratus they are twenty-four 

 This character alone would not have obtained a generic separation : 

 but a shortening of the neck, due not only to decreasing number 

 but to altered proportions of the cervical vertebras, when associated 

 with a well-marked modification of the teeth, of the sterno-coraco- 

 scapular frame, and of the paddle-bones, called for a separation of 

 the Sauropterygia into Plesiosaurus proper and a distinct genus, for 

 which the name Pliosaurus was proposed f, indicative of the nearer 

 approach which its species made to a generalized Saurian type. 



In the Crocodilia, for example, a common character of the teeth 

 is to have the usually simple conical crown, whether- finely ridged 

 or not, provided with a pair of enamel-ridges stronger than the rest, 

 and placed on opposite sides of the crown. 



In the genus Plesiosaurus the coronal ridges of the teeth are uni- 

 form or subequal, and the transverse section of the crown is circular 

 or sub-circular i. 



In the genus Pliosaurus, besides the shortness of the neck con- 

 comitant with hugeness of the head, a step towards the fresh-water 

 Saurians is made by the presence of a pair of coronal ridges, longer 

 and rather stronger than the rest, and rendered the more distinct by 

 the characters of the parts of the tooth-crown so defined ; the shorter 

 enamel-ridges being limited to one division, and this portion moreover 



* Owen's 'Paleontology,' 8vo, 1860, pp. 198, 209. 



t "Report on British Fossil Reptilia," part ii. in ' Reports of the British 

 Association for the Advancement of Science,' 1841. 



| "Monograph on the Sauropterygia," volume of the Palaeontographical 

 Society, issued 1865, pi. ix. figs. 3-9. 



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