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 

 Conida3, few of which reach any great depth. 



Among those Pleurotomida3 which would attract especial at- 

 tention is the exquisite Pleurotoma (Ancistro syrinx) 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- 



nse 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 



hundred fathoms, and has 



been found alive at Bar- 



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 



Fig. 282. Pleurotoma 

 (Ancistrosyrinx) ele- 

 gans. f. 



Fig. 283. Pleurotoma 

 subgrundifera. About |. 



