80 iiEPORT~1839. 



Plesiosauri, and it may be inferred that it had a large and heavy 

 head. 



As there are other species of Plesiosailrus from strata as re- 

 cent as those which contain the remains of the present^ and as 

 there are at least two species of Plesiosaiiri of gigantic propor- 

 tions, one of which has the humerus and femur of the ordinary 

 conformation, while the other has the same bones distinguislied 

 by large trochanterian processes, the aim which I have in view 

 to indicate as precisely as possible the different species of Ple- 

 siosaui'iis that have characterized our strata, seemed to be best 

 answered by giving a name to the present species indicative of 

 a structure which appears to be peculiar to it. 



Plesiosaurus costatus. 



Under this name I provisionally indicate a species character- 

 ized by a well-marked vertebra from the bone- bed of the lias at 

 Aust Cliff, near Bristol. 



In its general form and proportions, this vertebra (which is 

 is one of the anterior or middle cervicals) comes nearest to the 

 corresponding vertebra of the Plesiosaurus macrocephalus, and 

 presents a still more important character of agreement in the 

 contiguity of the costal and neurapophysial articular depressions. 

 I have already observed that the costal surfaces rise a little 

 above the level of the vertebra in the Plesiosaurus macrocepha- 

 lus : in the present specimens they are supported on two 

 distinctly developed articular processes, the intervening groove 

 of which is deeper and broader than in the Plesiosauri gene- 

 rally. The size of the two costal surfaces combined indicates 

 the hatchet- shaped rib to have been relatively larger than usual. 

 The free surface of the vertebra is more irregular than in the 

 PL macrocejjhalus ; it is marked in a characteristic manner by 

 irregular ridges near the raised circumference of the anterior 

 and posterior articular surfaces, from which rugged boundaries 

 deep and pretty broad grooves pass in a nearly parallel direction 

 towards the middle of the vertebra. The lower surface is tra- 

 versed by a strongly developed median longitudinal ridge, on 

 each side of which is the large characteristic vascular foramen. 

 The articular surface on the anterior and posterior sides of the 

 body deviates from the typical Plesiosaurian character in being 

 more concave in the centre than at the circumference, instead of 

 the reverse condition. About one fifth part of the articular sur- 

 face next the periphery is flat. The remaining median con- 

 cavity is slightly marked. 



The articular base of the neurapophysis is triangular, and the 

 two sides converge downwards at a more acute angle than I have 



