MOLLTTSCA OF THE ' CHALLENGER ' EXPEDITION. 439 



and oblique below ; on the last whorl they are sinuous, but very- 

 obsolete : the ordinary lines of growth are excessively faint, except 

 in the sinus-area, where they are sharp though minute. Spirals 

 — the suture is marginated below with a very small, prominent 

 gemmed thread ; below the plain suture-area is a slight keel, beset 

 by small tubercles where it crosses the riblets. On the body flat 

 threads are just perceptible, which increase in distinctness on the 

 point of the base ; on the snout these become more raised and are 

 parted by intervals of two or three times their width. Colour 

 porcellaneous white. Spire broadish, conical. Apex consists of 4 

 globose, conical, yellow embryonic whorls, whose point of union 

 with the regular whorls is slightly obscure ; the last is rather 

 closely curvedly ribbed longitudinally, while the earlier ones are 

 polished and smooth ; the extreme tip is small. Whorls 8 in all, 

 rather broad, with a very slight drooping shoulder defined by the 

 tubercled spiral thread and keel which bisects the earlier whorls, 

 but loses importance in all ways further down ; below this keel 

 there is a slight gradual contraction into the inferior whorl ; the 

 last whorl is very slightly tumid, with a protracted, slightly convex 

 base produced into a narrow snout. Suture a little impressed, 

 marginated below by the infrasutural thread, whose upper edge 

 forms a minute horizontal shelf, and which looks as if it girt-in 

 the shell*. Mouth long, narrow, pear-shaped. Outer lip thin and 

 sharp, steeply curved above, slightly convex below ; the edge ad- 

 vances below in a full round sweep ; above it forms a prominent, 

 but not very high shoulder, above which lies the open rounded 

 sinus, with a minute triangular shelf formed by the projection of 

 the infrasutural thread, and to a small extent separating the sinus 

 from the body. Inner lip narrow, shallowly excavated in the sub- 

 stance of the shell, dying out early in front on the oblique, sharp 

 but rounded, twisted and slightly reverted edge of the pillar. 

 H. 0-5. B, 0-18. Penultimate whorl, height 0*08. Mouth, 

 height 0'28, breadth 009. 



This species and P. (M.) macra, "Wats., belong to a group very 

 peculiar, and evidently numerous in the North Atlantic, as there 

 are young specimens from the same neighbourhood of five other 

 species, evidently all distinct, but stamped strongly with the same 

 features, of a thin glossy shell, obsolete sculpture, a slight gem- 

 mate keel, and the peculiar large, conically globose, minute-tipped, 

 smooth, longitudinally ribbed apex. 



* Hence the name. 



