524 Richard Owen^ Esq., on the 



The depressions above the costal surfaces for the lodgement of the bases of the neurapophyses 

 resemble in form those of the Plesiosaurus Hawkinsii, but extend further down upon the side of 

 the centrum. They are coextensive with the antero-posterior diameter of the vertebral body, and 

 are bounded by two lines meeting below, at a right angle. The angle formed by the corresponding 

 lines in the Plesiosaurus Hawkinsii is more open. The distance between these neurapophyseal 

 pits and the costal pits in the anterior cervical vertebrae, differs in different species of Plesiosaurus. 

 In the Macrocephalus the interspace is very short, never exceeding half the diameter of the costal 

 pit, even in the most anterior of the cervical vertebrae*. In the Plesiosaurus Haivkinsii the in- 

 terspace is equal to double the diameter of the costal pit in the corresponding vertebra. 



There may also be observed in the Macrocephalus an evident tendency in the surface supporting 

 the cervical vertebrae to rise above the level of the centrum : and this is the more interesting, as 

 in a large species of a yet uncharacterized Plesiosaurus nearly allied to Macrocephalus, (and of 

 which a figure of one of the posterior cervical vertebrae is subjoined) the surface on the centrum 

 and the corresponding surface of the neurapophysis do project, as short transverse processes ; 

 and thus approximate to the Crocodilian type. 



As the seventh, eighth, and ninth vertebrae happen to be displaced in the present specimen, 

 and their neurapophyses to be dislocated, the form and depth of the articular depressions for the 

 neurapophyses, as well as the canal for the spinal marrow, are thus brought into view. The cen- 

 trum presents only a plane surface for the spinal chord, the rest of the canal being completed 

 by the neurapophyses laterally, and the expanded base of the spine above. The surface in ques- 

 tion is bounded by two lateral curved lines, having their convexities turned towards each other. 

 Immediately below, and external to this surface on each side, are the deep and roughened pits 

 for tlie attachment of the neurapophyses. 



The cervical neurapophyses do not, in any of the Plesiosaurs, unite immediately together above 

 the spinal chord and canal, so as to form a continuous bony arch, spanning across that part, but 

 they stand upright from their sockets in the vertebral body, parallel with each other, or only 

 slightly converging at their superior extremities. They terminate above, in young individuals at 

 least, in broad rough articular surfaces parallel with the transverse axis of the vertebrae, but 

 sloping down from behind forwards with a slight sigmoid flexure at an angle of 25° with the 

 longitudinal axis of the vertebra. 



In the same way, therefore, as the rib or appendage to tJie transverse processes is bifurcate at 

 its proximal extremity, in those cases where the two transverse processes are separately developed 

 on each side of the vertebra, and where the rib is joined to both ; so here the spinal appendage 

 of the neurapophyses is bifurcate at its proximal extremity, and each fork rests upon the above 

 described oblique articular process of its own side. 



We have here an analogy between the lateral or costal and the superior appendages of the ver- 

 tebral centre which the Plesiosaurus alone has hitherto afforded. 



But besides the two surfaces developed for these articulations with the neurapophysis, each 

 fork of the spine sends off an articular or oblique process from its anterior and posterior extre- 

 mity ; the articular surface looking obliquely upwards and inwards on the anterior process, and 

 downwards and backwards on the posterior process: and thus the spines are locked together 

 throughout the whole vertebral column with the exception of the terminal vertebrae of the tail. 



* I have not as yet seen any Plesiosaurian cervical vertebrae that resemble those of the PL 

 Macrocephalus in this character, which is the more applicable and valuable from being presented 

 by the body of the vertebra alone. 



