MOLLUSCA OP THE ' CHALLENGER ' EXPEDITION. 449 



and are not so equably rounded ; the apex in that is a little 

 larger, the spirals are more regular and are waved, and are not 

 so much raised, and the longitudinals are much fainter. In De- 

 francia supercostata, E. Sin., the upper whorls are ribbed. What it 

 most resembles, however, is the Bela (?) expansa, Gr. O. Sars, who 

 kindly compared the two species and sent me his unique speci- 

 men for examination. That is a much longer and narrower shell, 

 of much faster increase, larger in the apex, and higher and nar- 

 rower in each corresponding whorl ; the whole spire is thus much 

 more elongated, and the last whorl is very much less tumid ; the 

 shell, too, is thicker, and the whole style of sculpture coarser. In 

 the same length it has about one whorl less. The curve of 

 each of the whorls is more tumid ; and as each slopes down to the 

 straighter suture, it slowly and slightly contracts. 



47. Plettrotoma (Thesbia ?) monoceros, n. sp. 



St. 104. August 23, 1873. Lat. 2° 25' N., long. 20° 1' W. 

 S.W. of Sierra Leone. 2500 fms. Mud. Bottom temperature 

 36°4. 



Shell. — Thin, ivory-white, high, narrow, drawn out, with fine 

 spiral threads, a very oblique impressed suture, rounded whorls, 

 and a contracted base produced into a longish snout. Sculpture. 

 Longitudinals — none but fine, somewhat unequal lines of growth. 

 Spirals — with the exception of the sinus-area, the whole surface 

 is covered by fine, rounded, unequal, and irregularly interrupted 

 spiral threads with rather broader intervals. Colour porcellaneous, 

 ivory-white, glossy. Spire remarkably narrow, high, drawn out 

 and conical. Apex broken. Whorls 5 remaining, but probably 

 8-9 in all, of very regular, but rather rapid increase, high, 

 oblique, slightly tumid : the last rounded, with a conical, pro- 

 tracted, but very lop-sided base running out into a longish straight 

 pillar and triangular snout. Suture very oblique and rather deeply 

 impressed. Mouth pear-shaped, scarcely pointed above, and pro- 

 tracted into a gradually narrowing canal below. Outer lip a very 

 regular curve in both its planes ; the outer edge has a very high 

 and prominent shoulder, whose upper side runs a long way parallel 

 to the body-whorl above it, having a very deep rather than narrow 

 sinus, between which and the body-whorl is no shelf whatever. 

 Inner lip is on the narrow body scarcely perceptible as a glaze ; 

 on the long direct twisted pillar it is a little thicker, but is very 

 narrow : the end of the pillar is cut off with a very long-drawn, 

 oblique, slowly narrowing, sharp, twisted edge. H. (of remaining 



