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 suneonteal 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 ; . 3°50 mm. 
Basal length 5 : . 850 mm. 
This form, which is more abundant a Lincoln than var. A, has a higher and 
blunter apex, the whole shell being more cap-like. Four or five conspicuous 
costee occupy the posterior area, whilst the indications of cost 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 costz 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 Lincoin, 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. Pitrotus Lavis, G. B. Sowerby, 1823. Plate XXVIII, figs. 15 a—c. 
1823. PriteoLus tavis, G. B. Sowerby. Genera of Shells, No. 19, figs. 5—8. 
9 
1823. — — J.Sow. Min. Conch., pl. eceexxxii, figs. 5—8. 
