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 1s even 
more fully and generally developed in the related group of 
Conide, few of which reach any great depth. 
Among those Pleurotomide which would attract especial at- 
tention is the exquisite Pleurotoma (Ancistrosyrina) 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 1s con- 
cave and carinated, the carı- 
næ 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 
Decir hr Meri hundred fathoms, and has 
"tam 2 ' been found alive at Bar- 
bados 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; 1 18 
Fig. 283. — Pleurotoma 
subgrundifera. About $. 
