TROCHUS. 379 



316. Trochus dimidiatus, Sowerby, 1817. Plate XXXI, fig. 8. 



1817. Tbochus dimidiatus, Sowerby. Min. Conch., pi. clxxxi, fig. 4. 



Description : 



Height . . . . .9 mm. 



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 Parhinsoni-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 d'Orb., fide Tawney, PI. XXXI, fig. 9. 



1852. Tbochus Zetes, d'Orbigny. Terr. Jur., vol. ii, p. 281, pi. cccxv, figs. 1 — 4. 

 1873. — — — Tawney, Dundry Gasteropoda, p. 32 (24), 



pi. ii, fig. 7. 



We do not gather from the text of the " Dundry Gasteropoda," whether the 

 specimen figured as T. 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 T. 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 T. dimidiatus. 



