TROCHUS. 379 
316. TRrocaus pimipiatus, Sowerby, 1817. Plate XXXI, fig. 8. 
1817. Trocuus prmipiatus, Sowerby. Min. Conch., pl. clxxxi, fig. 4. 
Description : 
Height é : : . . 9mm. 
Width : . 95 mm. 
Shell conical, smooth, without umbilicus. Spiral angle obtuse, from 70°—76°, 
spire rather less than half the total height. Number of whorls five, carinated 
above and below, concave between; sometimes the lower keel of the penult projects 
so as to produce a gibbous appearance. 
Body-whorl large, angular, and bicarinated ; base moderately full. Aperture 
subrhomboidal, with a tendency in some specimens to show a columellar furrow. 
Relations and Distribution.—Trochus dimidiatus is variable as to size and shape, 
though rarely exceeding 10 mm. in height. In general outline it resembles 
T’. spiratus, but has no spiral ornament beyond the very smooth keels. 
It is principally a fossil of the Upper Division. A very depressed variety 
occurs rarely in the Dogger. A variety occurs at Weldon; small specimens at 
Hook Norton; typical forms in the Parkinsoni-zone of the Cotteswolds, also at 
Midford and at Dundry. Rather more elongated specimens occur in the 
Parkinsoni-zone of Grove, &c. There is a curious form in my Collection from 
Dundry. 
As a possible variety of the above I draw attention to— 
Trochus Zetes dOrb., fide Tawney, Pl. XXXI, fig. 9. 
1852. Trocuus Zeres, d’Orbigny. Terr. Jur., vol. ii, p. 281, pl. ecexv, figs. 1—4. 
1873. _ _ = Tawney, Dundry Gasteropoda, p. 32 (24), 
pol oy, thes fe 
We do not gather from the text of the ‘‘ Dundry Gasteropoda,”’ whether the 
specimen figured as 7’. Zetes came from Dundry or from Bradford Abbas. In the 
Concavus-bed at the latter place, Trochus Zetes, so-called apparently on Tawney’s 
authority, occurs in considerable numbers (vide Fig. 9). The identification appears 
fairly correct; but the question naturally arises whether Trochus Zetes differs 
materially from 7’. dimidiatus. In the shells from Bradford Abbas the whorls of 
the spire are less angular, and there is a somewhat more marked umbilical excava- 
tion, the spire generally is less stumpy than in 7’. diiidiatus. 
