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FOSSIL REPTILIA OF THE 



Plesiosaurus homalospondylus (Tabs. V — VIII). 



In the year 1842 I examined, in the Museum at Whitby, Yorkshire, a collection of 

 Plesiosaurian vertebrae, which had been taken out of a heap of rubbish from the old 

 alum works carried on in the upper Alum Shale — a part of the Liassic series on that 

 coast, characterised by the Ammonites heterophyllus, Sow. 



The vertebrae were divisible into two groups, indicative of two species of 

 Plesiosaurus. 



Of one kind there was a series of sixteen consecutive cervical vertebrae, charac- 

 terised by the unusual concavity of the terminal articular surfaces of the centrum. On 

 making a section of two of these vertebrae cemented by the matrix in their natural 

 state of co-adaptation, the margins of the opposed articular surfaces were two lines 

 apart, showing the thickness of the inter-articular connecting ligamentous substance at 

 that part, while the middle of the articular surfaces left an interval of eleven lines, thus 

 approaching the ichthyosaurian type of vertebral union. 



The following were dimensions of the centrum of these cervicals. 



In. lines. 



Length .......... 1 9 



Breadth of articular surfaces . . . . . . Ill 



Height of ditto . . . 1 10 



The inferior surface of the centrum showed a median longitudinal convex ridge 

 between the two wide elliptical venous foramina. I named the species indicated by 

 these vertebrae Plesiosaurus ccelosponclylus,* in reference to the hollow terminal articular 

 surfaces. I hope to have, at a future opportunity, further means of illustrating this 

 species. 



The second series of vertebrae presented almost flat articular surfaces of the centrum 

 (Tab. V, figs. 3 and 6) ; the inferior surface was devoid of a median ridge, or had only 

 a slight rising (fig. 4, v) between the venous foramina, which were smaller and more 

 narrowly elliptical (ib., figs. 4 and 7) than in PI. ccelosponclylus ; the middle of the 

 surface was bounded laterally by the costal surfaces (ib., pi), and was nearly flattened, 

 being very slightly concave, both lengthwise and transversely. The costal surface is 

 of a narrow elliptical form, with the long axis parallel with that of the centrum ; the 

 dividing line or fissure is not conspicuous ; it is situated, as usual, rather nearer the 

 back than the front end of the centrum (Tab. V, fig. 2, pi) ; and a space more than 

 twice its vertical diameter intervenes between it and the neurapophysis (ib., np), or 



* KoiXos, hollow, air6\bv\os, vertebra. 



