338 GASTEROPODA OF THE INFERIOR OOLITE. 



At Lincoln I recognise two varieties, each of which differs more or less from 

 the Minchinhampton shell. 



Description, var, A (fig. 13) : 



Height . . . . .2*75 mm. 



Basal length .... 6*50 mm. 



Shell suborbicular, with a sharp subcentral apex of low elevation. The 

 radiating ribs are about eighteen in number, the four posterior ones being more 

 elevated and wider apart than the others. The indications as to the base are not 

 good, so that one cannot say whether the margin is serrated or otherwise. 



This form is much more depressed and of smaller habit than the one which 

 prevails in the Great Oolite, whilst the ribs are fewer in number and relatively 

 larger. 



Var. B (fig. 14) : 



Height ..... 3*50 mm. 

 Basal length .... 8*50 mm. 



This form, which is more abundant at Lincoln than var. A, has a higher and 

 blunter apex, the whole shell being more cap-like. Four or five conspicuous 

 costse occupy the posterior area, whilst the indications of costse in the anterior 

 portion of the shell are usually indistinct. A few coarse serrations are noticeable 

 in the posterior portion of the margin, the rest of which is smooth. 



In height and shape var. B resembles the typical P. plicatus, but the costse are 

 less numerous. There are also some indications of difference in the base, the 

 position of the cushion not being exactly the same. So far as I can ascertain the 

 inner lip is not crenulated, as stated by Sowerby ; Morris and Lycett make no 

 mention of this feature. 



Relations and Distribution. — Pileolus plicatus may be accepted as a sort of 

 generalised term for costate forms of the genus. Strictly speaking, I consider 

 that there are two forms at Lincoln, which so far differ from the regular Bathonian 

 P. plicatus as to be possibly entitled to rank as two distinct species. But, in 

 order to establish these, more favorable conditions for comparison are required. 



The Pea-grit horizon of the Cotteswolds and the " Base bed " at Lincoln are 

 the only places in the Inferior Oolite of this country where Pileolus plicatus and 

 its varieties are known to occur. 



271. Pileolps ltevis, G. B. Sowerby, 1823. Plate XXVIII, figs. 15 a— c. 



1823. Pileolus LiEVis, G. £. Sowerby. Genera of Shells, No. 19, figs. 5 — 8. 

 1823. — — J. Sow. Min. Conch., pi. ccccxxxii, figs. 5 — 8. 



