196 
transverse ribs that cover the other part of the shell abruptly 
terminate. Length and breadth nearly equal. 
A frequent shell in Great Britain, particularly in the 
inferior Oolite, as at Little Sudbury Wilts, and at Oxford. 
I have received specimens from the Rev. H. Steinhauer 
and Mr. Sheffield. Neither is it rare on the Continent. 
The specimen figured shows the kinge very distinctly, and 
also the single muscular impression. It is nearly a solid 
gray Carbonate of Lime, and being found in an ochraceous 
matrix, is more or less colored with ochre. Little parasites 
have formed holes, &c. in most of these shells; sometimes 
they have vermicnli about them. 
. TRIGONIA spinosa. 
TAB. LXXXVI, 
Spec. Char, Roundish, depressed, many-ribbed; 
anterior side truncated ; ribs oblique, diverging 
both ways from the ridge that separates the an- 
terior side; set with short spines. 
Syn. Park . Org. Pern. 3. T. 12.. F. 7 . 
The ribs of this shell are composed of tiled plates, the 
edges of which, are twisted up into a sort of flat spines ; 
they are small at their commencement, but increase in 
thickness as they descend obliquely from the ridge towards 
the front; they are curved, and terminate very suddenly at 
the edge. The disposition of these ribs, and the regularity 
of their asperities, produces a considerable degree of novelty 
and elegance of feature. 
This very curious species is from Blackdown : the casts 
are siliceous; it conveys, even when magnified, a sort of 
deception to the sight, and it lias its name from that look, 
which when thoroughly understood, does not appear accu- 
rate, and may be the rather unfortunate, as it is likely 
better to suit some other species hereafter, but the original 
name must be retained. 1 am glad to show the hinge, by 
favor of the Rev. H. Steinhauer. This shell has been pene- 
trated by some curious parasites. 
