122 PAPERS, ETC. 
rises a thick triangular septum, on either side of which 
is a small sub-eircular ridge, formed by a range of granu- 
lations. 
Obs. This species has been figured by Mr. Davidson and 
M. E. Deslongchamps, the latter of whom mentions that 
the sub-circular ridges in the French specimens are some- 
times formed of a double line of granulations. In exam- 
ples that have come under my observation, the ridge is 
usually made up of single granulations, and those at times 
widely separated; but I have no hesitation in placing them 
under this species. It is common in the inferior oolite ot 
Dundry; the large valve is found in abundance attached to 
oyster and other shells, in the fuller’s earth near Combe 
Down, Bath; it occurs in the great oolite of Hampton 
Down, and I have obtained it in the middle lias of 
Ilminster. 
