64 FOSSIL REPTILIA OP THE 



from the Plesiosaurus constrictus, from the Chalk of Steyning, in Sussex. Although 

 the sutures connecting the neural arch with the centrum are traceable, there has been 

 a certain degree of anchylosis, which has helped to maintain the arch in its natural 

 connection, notwithstanding the degree of pressure and distortion to which the whole 

 vertebra has been subject. Each neurapophysis, which measures one inch five lines 

 in antero-posterior diameter at its narrowest part, is smoothly rounded off at both its 

 free borders, of which the anterior one is the thickest ; the posterior zygapophysis is 

 developed at rather more than an inch above the base of the neurapophysis ; its flat 

 oval articular surface looks downwards, and a little outwards : the neural canal is 

 relatively wider than in the Plesiosaurus Bernardi, and its area is oval, with the great 

 end downwards. The spinous process, of which nearly four inches is preserved, has 

 an antero-posterior diameter at its base, of nearly two inches, and is strengthened 

 behind by two buttress-like ridges, which rise converging from the summit of each 

 posterior zygapophysis : bounding an angular depression at the back part of the spine, 

 as in the Plesiosaurus Bernardi, and many other species. The total height of this 

 vertebra, as far as the spine is preserved, is seven inches and a half, and the total 

 length of the Plesiosaurus, to which it belonged, was probably not less than sixteen 

 feet. There are preserved in the same block of Chalk with the vertebra above 

 described, the summit of the neural arch, with the base of the spine of another 

 vertebra, and a portion of one of the long ribs of the thorax, fig. 1, pi. 



Plesiosaurus pachyomus, Owen. T. XX, XXI. 



' Report on British Fossil Reptiles,' Trans. Brit. Association, 1839. 



This species of Plesiosaurus was founded on certain remains discovered in the 

 Upper Green-sand at Reach, about six miles from Cambridge, and placed by the 

 Rev. Professor Sedgwick in the Wordwardian Museum of that University. 



The specific name " pachyomus"* relates to the unusual thickness of the humerus, 

 the distal flattened end of which is one inch and a half thick, the breadth of the 

 same part being only four inches and a half, and the length of the entire bone nine 

 inches and a half. The contour of the articular head is transversely oval. The 

 central part of the bone is occupied by a coarse cellular structure, one inch and a 

 half in diameter, surrounded by dense osseous walls, three lines thick. 



In the rich and instructive collection of Reptilian fossils, from the Cretaceous 

 deposits in Cambridgeshire, in the possession of James Carter, Esq., M.R.C.S., of 

 Cambridge, there are several vertebral bodies or "centrums" of the same species o 

 Plesiosaurus which show the change of proportion in the breadth and depth of the 



* W.a\vs, thick, Sfios, humerus, or arm-bone. 



