ATAPHRUS. 353 
the last-named species. It is true that d’Orbigny described his shell as being 
formed of a regular angle, whereas our shells have an obtuse apex with a mean 
spiral angle under 50°. But on examining a series of Normandy specimens (Sully) 
referred to Trochus Acis, I find that the apex is in all cases extremely obtuse. 
Hence it would seem that in this respect, d’Orbigny’s figure is misleading. 
The Bristol Museum contains several specimens from Dundry, which Mr. 
Tawney referred provisionally to T'rochus Ibbetsoni, Morris and Lycett (‘ Great 
Oolite Moll., pt. 1, p. 62, pl. x, figs. 4, 4a). At the same time he remarked 
that their texture and aspect seemed to connect them with Monodonta levigata, 
Sow., a supposition which there is little doubt was correct. The question as to how 
far Trochus [bbetsoni and Ataphrus Acis represent the same species still remains for 
consideration. 
Description : 
Average height about : : : 12 mm. 
Width about : : : : 10 mm. 
Mean spiral angle about ; 50°. 
Shell obtusely trochoid, smooth, rather AveRe number of whorls five or six. 
The body-whorl is about half the entire height, int slightly convex, and making 
a blunt angle at the periphery. Base nearly flat; aperture small and circular, with 
a slight denticle towards the end of the columella, succeeded anteriorly by a shallow 
pit or furrow. 
Relations and Distribution.—This is the narrowest and most trochoid of all 
the smooth ‘‘ Monodontas.”. It also shows a feebler development of the denticle— 
the “tubercle” of Gemmellaro. By a widening of the spiral angle it passes into 
Ataphrus Acmon, and there are intermediate forms. Afaphius Acis is essentially a 
fossil of the Upper Division of the Inferior Oolite, occuring somewhat rarely at 
Burton Bradstock, and also at Dundry. Typical forms are probably not found 
outside the Dorset-Somerset district. 
Variety in the Lincolnshire Limestone (Trochus Ibbetsoni, M. and L.), height 
about 7mm. ‘These small shells, chiefly from Weldon and Ponton, are not in good 
preservation. The proportions are about the same as in Ataphrus Acis, of which 
species they are most likely a variety reduced in size, as is the case with so many 
of the Gasteropods of the Lincolnshire Limestone. I think that analogous repre- 
sentatives of At. Labadyei and At. Acmon also occur in this formation. 
N.B.—Morris, in his paper on the ‘ Lincolnshire Oolites”’ (‘ Quart. Journ. Geol. 
Soce.,’ vol. ix, p. 326), quotes Trochus Acis, and not Trochus Ibbetsoni, from Ponton. 
On the other hand, Mr. Etheridge (Judd, ‘Geology of Rutland,’ p. 283) gives 
Trochus Ibbetsoni 1m his catalogue of: Lincolnshire Limestone fossils, and not 
Trochus Acis. 
