66 



THREE CRUISES OF THE " BLAKE. 



the aperture with the outside of the preceding whorl. This 

 notch permits the refuse matters discharged from the anal open- 

 ing to escape outside of the shell without fouling the water 

 which is used by the gills in respiration. These mollusks are 

 found at all depths, are animal feeders, and some of them are 

 provided with barbed hollow teeth, having a duct to which a 

 gland supplies a poisonous substance ; such an apparatus is even 

 more fully and generally developed in the related group of 

 Conidse, few of which reach any great depth. 



Among those Pleurotomidse which would attract especial at- 

 tention is the exquisite Pleurotoma (Ancistrosyrinx) elegans 

 (Fig. 282), one of the most beautiful gems of the sea. It grows 

 to an inch and a half in 

 length, and is of a light 

 straw color; the posterior 

 surface of the whorls is con- 

 cave and carinated, the cari- 

 nas being delicately fringed 

 with sharp triangular 

 points ; it has a deep notch, 

 which in perfect specimens 

 has a raised margin. This 

 species descends to eight 

 Fig. 282. — Pleurotoma hundred fathoms, and has 



(Ancistrosyrinx) ele- , £ , r ^ Fig. 283. — Pleurotoma 



gans . ? . been found alive at Bar- su ^ undifera . Aboutf . 



b a d o s in seventy - three 

 fathoms. Its fossil allies extend as far back as the eocene. 

 Pleurotoma subgrundifera Dall (Fig. 283) is a form which, in- 

 stead of having the margin turned toward the tip of the spire, 

 has the sharp keel bent in the opposite direction toward the 

 canal, like the edge of an umbrella. Another pretty species, 

 dredged in deep water both by the " Blake " and the " Challen- 

 ger," is Pleurotoma Blakeana ; and still another, short and 

 stout, with delicate reticulate sculpture, has also been obtained 

 by the Fish Commission, the P. curta of Prof. Verrill. Both 

 these resemble in shape the Belas of the arctic seas. A very 

 elegant and widely distributed little shell is the P. limacina, 

 polished, smooth, with a beaded garland at the suture ; it is 



