Structure of the Plesiosaurus Macrocephalus. 527 



a small part of the costal articular surfaces being contributed by the centrum ; 

 these surfaces beginning to sink down again upon the bodies of the vertebrae, 

 where the increased breadth of the ribs indicates that they gave attachment 

 to the pelvic arch. 



Guided by this character in reference to the PL Macrocephalus, I find that the costal articular 

 surface begins to descend upon the vertebral body at the fiftieth vertebra (counting from the atlas), 

 and consequently reckon this and the succeeding vertebrae as sacral ; and their relative position 

 to the dislocated ilium verifies the supposition that the same character, with reference to the 

 costal articular surface, points out the bodies of the sacral vertebrae in the PL Macrocephalus, as 

 it does those of the PL Hawkinsii. 



It is at this part of the vertebral column, in Lord Cole's specimen, that the separation of the verte- 

 bral elements from each other, and the displacement of the bodies, again bring into view the surface 

 which supported the spinal marrow, and the articular depressions for the neurapophyses on each side. 



The surface which supported the spinal marrow is small and flattened, slightly impressed, 

 bounded by two gently-curved lines, whose convexities are turned towards each other. 



A comparison of the medullary canal at this part and at the cervical region, shows that the 

 usual law of the increase of the spinal chord at the parts where large nerves were required to be 

 given off to supply the locomotive extremities obtained in the extinct Enaliosaurians, as in their 

 existing congeners. 



The spinous processes are locked together by double oblique articulating surfaces, as described 

 in the cervical vertebrae. 



Of the Tail. 



Of the tail, there remain in the present specimen the bodies of eight vertebrae, with a few of the 

 caudal ribs and haemapophyses. From their position, and the decreasing size of the vertebrae, we 

 may safely judge that the tail had the usual proportions observable in other species of Plesiosaurus. 



Each of these centres presents on its upper surface the continuation of the spinal canal, bounded 

 by two lateral curved lines, with their convexities turned towards each other ; and on each side of 

 the canal a roughened subtriquetral surface for the attachment of the base of the neurapophyses. 

 Below this, on each side of the body, there is a rounded pit for the articulation of the caudal rib. 



These differ from the costal pits in the cervical vertebrae, in not being traversed by a groove. 



The inferior surface of the centrum pi-esents a slightly concave, transversely oval surface, with- 

 out any longitudinal ridge ; and on each side of the anterior and posterior margins there is a half- 

 articular surface, designed to form, in apposition with the adjoining vertebrae, a point of attach- 

 ment for the haemapophyses. 



The length of that which appears to be the eighth caudal vertebra, is, to its transverse diameter, 

 as 2 to 5. 



In the PL Hawkinsii, I find that the rib has left the neurapophysis, and slid down wholly upon 

 the centrum at the sixty-first vertebra. 



In a very perfect tail of a Plesiosaur in Mr. Hawkins's collection, the haema- 

 pophyses are developed beneath the twenty-eight terminal caudal vertebrae, 

 the two or three last small ones only excepted. The bodies of these small 

 terminal caudal vertebrae also present an interesting modification of the sur- 

 faces by which they are attached to one another, and if found detached, would 



