370 GASTEROPODA OF THE INFERIOR OOLITE. 



1. Umbilicated species with a somewhat convex base. These forms seem to 

 establish a link with the Delphinulas just described. 



2. Species with a nearly flat base, and mostly without umbilicus == Zizyphinus. 



3. Trochiform shells without umbilicus, some of doubtful nature. 



N.B. — There is a group of small umbilicated Trochi belonging to the first 

 division, which I might designate the Sanders w-group. It so happens that Mr. 

 Tawney founded this species on a single and rather imperfect specimen from 

 Dundry, where probably the form is very rare. In Dorsetshire, however, there is 

 a somewhat numerous group of shells, possessing a general resemblance to the 

 type of T. Sandersii, though passing through numerous varieties into something 

 very different. The three following named forms are placed in this group. 



306. Trochus Sandersii, Taivney, 1873. Dorset variety, Plate XXXI, fig. 1. 



Cf. Trochtts Sandersii, Tawney. Dundry Gasteropoda, p. 31 (23), pi. ii, fig. 4. 



Description : 



Height . . . . .14 mm. 



Width . . . . .15 mm. 



Spiral angle ..... 68°. 



Shell conical and moderately umbilicated. Spire about half the total height 

 and rather obtuse towards the apex. Number of whorls about six ; those of the 

 spire are concave and marked off by a very wide suture. At the posterior margin 

 of each whorl is a circlet of fine tuberculations, whilst the anterior margin is 

 keeled and marked with a circlet of larger tuberculations ; radial costse cross the 

 intervening hollow. 



The body-whorl is concave above and bicarinate below, the upper carina being 

 the most salient; the ornaments are similar in character to those of the spire; 

 base moderately convex with decussated ornamentation ; umbilicus deep and 

 somewhat funnel-shaped and encircled by a fringe of tubercles. Aperture sub- 

 rhomboidal. 



Relations and Distribution. — The single specimen described by Tawney from 

 Dundry is narrower than the majority of the Dorset shells referred to this species. 

 Nevertheless, these differ among themselves to such an extent that it is not easy 

 to say what the type should be. In Dorsetshire T. Sandersii, i. e. the fossils 

 referred to this species seem to occur on a low horizon, chiefly in the Murchisonse- 

 zone or at the base of the Concavus -bed. 



