KIMMERIDGIAN PLIOSAURS. 163 



portion of the skeleton, save that of the scapular clement, are common to both 

 the genera of the Sauropterygia. 



The chief and suggestive modification of the mass in the Pliosam^ian genus is 

 the retention of a typical character of the scapula which is lost in Plesiosaurus, the 

 production, namely, of the part of the blade-bone (PI. 20, fig. 2, si x) latcrad and 

 dorsad, where it terminates freely. This portion represents the main body of the 

 scapula in higher vertebrates ; but, as in the allantoic or abranchiate group,^ 

 (Beptilia, Aves) without expanding. The portion of the scapula common to both 

 the present extinct genera, which contributes its share (h) to the glenoid cavity, is 

 separated, in Pliosaurus, from the free portion (51 ^r) by the notch (n). In advance 

 of this the Pliosaurian differs from the Plcsiosaurian scapula by its greater relative 

 breadth, extending its sutural border (sA) mesiad so as to touch or join the fore 

 end of the coracoid (so). 



The coracoids retain their large proportional size, but have a less even or 

 flattened outer surface ; mesially they bidgo to their common suture (s, s), giving 

 more room to the ventral cavity ; and, at the transverse parallel with the borders, 

 which they contribute to the vacuities (c s,cs), they bend dorsad, or inward, suddenly 

 contract (sc) ; but contribute, as in Plesiosaurus, the mesial border of those 

 vacuities, and articulate, underlapping it, with the hinder end of the episternum (59'). 

 In thus determining the homologies of the constituents of the complex bony 

 buckler in 8auropteri/gia, I have exhausted every subject of comparison at my 

 command in other Reptilian forms, both fossil and recent. The degree in which 

 the abdominal surface is defended by bone in Sauroptcrygia resembles that in 

 Ghelonia. But, as I have elsewhere^ shown, by dermal ossifications chiefly, these 

 not answering to the endoskeletal elements which have been modified to that 

 end in the subjects of the present section. 



Pliosaurus pobtlandicus, Owen. Smiropterygia, PI. 19, fig. 8. 



The generic distinction from Plesiosaurus, indicated by the term Pliosaurus, and 

 originally suggested by characters of the teeth and the cervical vertebrse, is further 

 confirmed by the structure of the bony framework of the paddles. The modifica- 

 tion in question, like the fore-and-aft compression or shortness of the cervical 

 centrums, exemplifies the nearer resemblance, I will not say affinity, of Pliosaurus 

 to Ichthyosaurus ; the segment of the natatory limb which answers to the anti- 



1 ' Anatomy of Vertebrates,' vol. i, 1S66, pp. 6, 7. 



2 'Philosophical Transactions' for 1849, "On the Development and Homologies of the Carapace 

 and Plastron of the Chelonia. 



