UNIVALVE UNILOCULAR SHELLS. 243 



elevated band at the upper part of each whirl ; but it is smoother 

 than his shell. It is often four inches, or upwards, iu length. Shells 

 of the same form are found, without any appearance of a band at the 

 sutures.— No. 11 is a more elongated shell, neai'ly corresponding with 

 the turritella conoidea of Sowerby, Tab. 51, Fig. 1, 4; or his t. 

 elongata, Fig. 2, obviously a variety of the same species. There is a 

 slightly elevated thread at the suture, on the upper side of each whirl ; 

 but in many specimens the thread is scarcely, if at all, discernible. 

 Other shells of this form resemble Sowerby's turritella edita. Tab. 51, 

 Fig. 7. Almost all these shells are finely striated across, that is, 

 spirally, in the direction of. the whirls. — Shells of the same form 

 occur, but rarely, in the dogger; where also we find others resem- 

 bling No. 8. Minute shells, like the latter, are seen occasionally in 

 the lias and pyritous nodules. 



Trochus, Top-shell. Shells of this genus occur in the oolite, 

 the grey limestone, the blue limestone, the dogger, and the Staiths 

 band. No. 4, PI. XI, from the blue limestone, resembles t. zizyphi- 

 nus, but is differently marked; and having the appearance of an 

 umbilicus, it may be more akin to t. radiatus. The granules of the 

 numerous transverse striae are connected by slender threads. The 

 apertiu-e is rhomboidal ; and the base is flat, or a little convex, and 

 marked with concentric lines. The markings in the specimen 

 are somewhat worn off. We found also in the blue limestone^ 

 an imperfect cast of a large trochite of the same shape, above three 

 inches in diameter, with an oblique or spiral umbilicus. — No 1, from 

 the Malton oolite, resembles t. magus, being a flat shell, striated 

 transversely, and with the whirls plaited on their upper margins. It is 

 near two inches broad, and little more than one inch deep. 



Fig. 6 represents the cast of a large shell from the grey limestone, 

 which has measured nearly four inches in breadth, and above two 

 in depth. Though resembling a trochite, particularly t, concavus, it 



