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 Sandersii-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 7. Sandersii, though passing through numerous varieties into something 
very different. The three following named forms are placed in this group. 
306. Trocnus Sanpersu, Tawney, 1873. Dorset variety, Plate XX XI, fig. 1. 
Cf. Trocuus Sanperstt, Zawney. Dundry Gasteropoda, p. 31 (23), pl. ii, fig. 4. 
Description : 
Height ; : . : . 14mm. 
Width ; ; : : Selon mam: 
Spiral angle . : j 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 coste 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 7. Sandersii, i.e. the fossils 
referred to this species seem to occur on a low horizon, chiefly in the Murchisonx- 
zone or at the base of the Concavus-bed. 
