ON BRITISH FOSSIL REPTILES. 159 



greensand indicated by bones of the extremities, that of the Maidstone 

 greensand by its teeth, and that of the Wealden formation recognized by 

 its vertebrae, are all parts of the same extinct reptile. 



Genus Rysosteus, nob. 



I have been favoured by Mr. Johnson of Bristol, with the opportunity of 

 examining a small anterior dorsal vertebra, (No. 177, of his interesting Col- 

 lection,) half imbedded in its pyritic matrix, from the Bone-bed of Aust Pas- 

 sage, near Bristol. 



Both articular ends of the body of this vertebra are concave, but deeper 

 than in Teleosaurus, with a central short transverse linear impression. The 

 lower part of the side of the body is raised into an obtuse longitudinal ridge, 

 above which, between it and the transverse process, is a wide but not deep 

 depression. The centrum slightly expands to the two extremities, which have 

 a circular contour. The transverse process is broken off: its base, which is 

 as deep as long, rests in a small proportion upon the centrum, but chiefly upon 

 the side of the neurapophysis, the limits of which are not defined by a per- 

 sistent suture. The neural arch rests upon the whole antero-posterior extent 

 of the upper part of the centrum, rises nearly vertically to a height not quite 

 equal to that of the centrum, then slopes abruptly inwards to support the base 

 of the spine. This is nearly equal in antero-posterior extent to the centrum, 

 and slightly increases in that direction by inclining over the interspace of the 

 posterior oblique processes : it also slightly gains in thickness, and is termi- 

 nated by a flat and rough surface, the contour of which is nearly parallel with 

 that of the under surface of the centrum. The sides of the spine for two lines 

 below the summit are wrinkled* or impressed by vertical or slightly oblique 

 coarse grooves. The posterior oblique process is moderately long and slender ; 

 its flat elliptical articular surface looks downwards and slightly outwards. The 

 non-articular surfaces of the vertebra are smooth, except near the summit of 

 the spine, the lateral ridges and grooves of which form the chief characteristic 

 of the present vertebra. 



This vertebra, though it resembles those of a few species of Plesiosaur in 

 the depth of its terminal articular surfaces, difl'ers too much in its length and 

 lateral compression to be referable to that genus ; the rough and thick trun- 

 cated summit of the spinous process rather indicates the species to have be- 

 longed to the loricate family of Saurians. It diflPers from the vertebrae of the 

 Teleosaur and other known Amphicoelian Crocodiles in the form and verti- 

 cal thickness of the transverse processes, in the lateral longitudinal ridge of 

 the centrum, and in the antero-posterior extent and form of the spinous pro- 

 cess. It difl'ers from the vertebrae of the Labyrinthodon in the articidar ends 

 being at right angles to the axis of the centrum and not oblique ; in the greater 

 vertical thickness of the transverse process ; and in the spine not being sud- 

 denly expanded and flattened at the summit, as in the dorsal vertebrae of the 

 Labyrinthodon. 



The following are dimensions of the vertebra of Rysosteiis : — 



Lines. 



Antero-posterior extent of centrum 11 



Transverse diameter of articular end 5 



Vertical diameter of articular end 6 



Vertical diameter of entire vertebra 16 



Antero-posterior extent of spinous process .... 10 



* The name here proposed for the Saurian of the Bone-hed, from pucriis wrinkled, ocreov 

 a bone, relates to this structure. 



