1 38 ON GENERIC CHARACTERS IN THE ORDER SAUROPTERYGIA. 



bone." The true homology of the constituents of the sterno-coraco- 

 scapular frame is yielded by the endo-skeleton of Chelonia, in which 

 the true coracoid is the largest and broadest of the elements of the 

 shoulder-arch, the mesial margins almost meeting beneath the fore 

 part of the thoracic-abdominal cavity. 



Recalling the impression made on the mind of Cuvier * by the 

 first account of the Plesiosaurus dolieJiodeirus, we can appreciate 

 the advantage and reward of continued researches, in the blotting- 

 out of seeming anomalies, and in the addition of features of affinity 

 linking on the strange extinct form (brought to light by Conybeare) 

 to the general Eeptilian type, and diminishing the intervals which 

 seemed to exist in the series. 



The few and short cervical vertebrae in Pliosaurus manifest, with 

 these Ichthyosaurian proportions, by their amphicoelian, almost 

 flattened, articular surfaces, characters of contemporary Crocodilia, 

 of which Reptiles we are also reminded by the short and thick 

 neck, the large head, and the powerful jaws of the later-found 

 Sauropterygian genus. The teeth, moreover, are now implanted in 

 distinct sockets ; and the blade-bone resumes its normal character. 



It is interesting to note that the species tending to diminish the 

 interval that seemed to separate Conybeare's Plesiosaurus from the 

 typical Reptilia have been hitherto obtained from Hesozoic deposits 

 less ancient than the Lias. All my evidences of Pliosaurus have 

 been derived from Kimmeridgian and Portlandian beds. 



A third generic modification of the Sauropterygia is indicated by 

 teeth and a portion of the skull from a part of the Cretaceous series ; 

 but I wait for further acquisition of its remains before submitting 

 to the Society the differential characters of the genus Polyptychodon. 



Discussion. 



The President stated that great differences of opinion existed as 

 to the form of the scapular arch in Plesiosaurus. He himself believed 

 that there is evidence that in Pliosaurus there was a dorsal pro- 

 longation of the scapula similar to that found in the higher ver- 

 tebrates. 



* • (Jest par cet habitant de l'ancien monde, peut-etre le plus heteroclite, et 

 celui de tous qui paroit le plus meriter le nom de monstre, que je terminerai 

 cette histoire des animaux perdus."— Ossem. Foss. torn. v. pt. 2, p. 476, 1824. 



