4 BRITISH FOSSILS. 



There are three principal forms conspicuous among these varieties, 

 which, if separated from each other in collections, might seem specifi- 

 cally distinct, but between which every gradation of transition may be 

 found. The first of these is the more or less regular sub-depressed 

 hemispheric form, most common, and apparently the normal one. Its 

 most convex state closely approaches the conical variety, which is repre- 

 sented in the original figure of Discoidea hcemisphcerica. This, on one 

 side, passes into a small very convex variety, having the anus more than 

 usually marginal, in which state it appears to constitute the Holectypus 

 clevauxianus of Cotteau, with whose figures and descriptions we have 

 English specimens perfectly according. On the other side it becomes 

 more and more depressed, until we get it only gently convex on the 

 back. Specimens of this kind seem to have constituted the Discoidea 

 marginalis of M'Coy, said to be " very abundant in the inferior oolite 

 of Bridport," and respecting the identity of which with the species here 

 described I entertain not the slightest doubt. In order to show the 

 variations of dimensions in this species, I give the following measure- 

 ments of ten Dorsetshire specimens in the collections of the Geological 

 Survey : — 



No. 



Locality. 



Breadth, 



Height. 



Anus to 

 Mouth. 







Inch. 



Inch. 



Inch. 



1 



Stoke Knaps (j3) . . . 



1 







2 



Loders 



It's 





0?| 



3 



Crewkerne .... 



1^5 



OtI 





4 



Crewkerne .... 





Oil 



Ofl 



5 



Stoke Knaps .... 



It^i 



Oil 





6 



North Nibley . . . 



111 



ofl 



Ofl 



7 



Castle Gary (y) . . . 



OtI 





Of, 



8 



Stoke Knaps (7) . . . 



Of. 



Of, 



Of, 



9 



Shipton Gorge 



0^2 



0^ 



Ofl 



10 



Bridport Harbour (7) 



Of, 







Locality and Geological Position. — During the examination of the 

 Inferior Oolite strata in Somersetshire and Dorsetshire by the mem- 

 bers of the Geological Survey, this species was collected abundantly, 

 chiefly in the sands of the Inferior Oolite, in numerous places, associated 

 in most instances with Dysaster ringens and often Dysaster hicordatus. 

 The following is a list of the principal localities in which it was found : 

 Hazelbury, Crewkerne, Lyttelton Hill near Cadbury, Whatley near 

 Frome, Little Windsor, Loders (top beds). Stoke Knaps, Greenland, 

 Compton Pauncefoot (bottom beds), Pilcombe, Bruton, Shipton Gorge, 



