1873.] SEELEY — CETAETHE0SATJET7S WALKERI. 505 



Mr. Evans pointed out that the main question at issue was 

 whether the sandy beds below the Cephalopoda-bed of Bradford 

 Abbas were really the equivalents of those which were found above 

 the analogous bed in Gloucestershire. 



Prof. Ramsay regretted the absence of Fellows more especially 

 interested in this question. For himself, he considered it impossible 

 to correlate particular zones over any large area, and thought that 

 the whole series might be regarded as passage-beds, the order of 

 which might vary even within a limited distance. 



4. On Cetaetheosat/etjs Walkeei (Seeley), an Ichthyosatjeian 

 from the Cambridge Uppee Greensand. By H. G. Seeley, Esq., 

 F.L.S., F.G.S. 



The specimen now described was discovered several years since by 

 J. F. Walker, Esq., M.A., F.G.S. , among some fossils gathered at a 

 coprolite-washing in the Upper Greensancl, from near the railway- 

 bridge at Ditton, N.E. of Cambridge. Mr. Walker recognized the 

 specific importance of his fossil ; and, from a cast, I made a brief note, 

 enrolling the species in the Cambridge Greensand fauna as Ichthyo- 

 saurus Wallceri *. No other bone presumably referable to the same 

 species is known to have been found ; and, as with many of the 

 associated fossils, abrasion has done its work upon this femur in a 

 way to suggest that, like many disfigured recent bones to be picked 

 up on our own shores, these Greensand exuviae were rolled on a 

 pebbly beach before deposition in the bed of phosphatic nodules at 

 the base of the deposit. 



The bones of the extremities of Ichthyosaurians, as was pointed 

 out by Mr. Hawkins, afford excellent characters by which species 

 maybe defined ; but in this ordinal group no sufficient description of 

 the skeleton has been made to assist comparison of specimens with 

 a type, perhaps because the varieties of structure in the different 

 genera confounded under the name of Ichthyosaurus are such as 

 to make a comprehensive diagnosis of the several bones a task 

 of difficulty. 



" The femur of Ichthyosaurus is a strong short bone, with a small 

 compressed distal end, having its greatest extension at right angles 

 with the greatest width of the head. The distal end shows two or 

 three articular facets for the bones of the foreleg. The proximal 

 end is large, convex, and broad, and sends off on each side a tro- 

 chanteroid process, which makes the head massive. These processes 

 are rounded laterally, and extend down the short shaft, gradually 

 being obliterated with the increasing compression of its distal end. 

 The outer side of the bone is the natter ; the inner side is made 

 convex by a more or less well-defined, rounded, longitudinal ridge, 

 which, extending from the head of the bone, slightly curves convexly 

 backward, so as to indicate that the greater part of the head lies 



* Aves, Ornithosauria, and Reptilia (1869, 8vo), p. 64. 



