372 
MOLLUSCS. 
a small aperture, notched in front for the passage of the siphon. The whorls are 
flat and generally divided below the suture by a spiral furrow. They are 
longitudinally fluted, smooth or noduled, and it is upon these differences in 
sculpture that the various subgenera are founded. 
The third family— Pleurotomidce —contains an enormous number of species, 
certainly more than a thousand having been described. They are mostly small, 
and show great variety in form. The species of the typical genus Pleurotoma are 
spindle-shaped, that is, have a long tapering spire at one end, and a prolonged 
beak or canal at the other, and the outer lip has a distinct slit somewhat below 
the suture. In Surcula and Drillia the slit or notch is at the suture, in Bela it is 
indistinct. Some forms, Clavatula and Pusionella, have a semi-ovate operculum, 
with the nucleus lateral; in Pleurotoma, Drillia, etc., it is ovate-pyriform, with a 
terminal nucleus; whilst in Mangilia, Cythara, etc., it is altogether absent. 
Columbarium, containing only a few species, is remarkable for the great length 
of the anterior canal, and the spine-like ornamentation of the whorls. C. pago- 
doides, dredged off Sydney in four hundred and ten fathoms, is one of the most 
beautiful of all the family. All degrees in the length of the canal are observable, 
until we find it reduced to a mere notch at the base of the aperture. Pleuroto¬ 
midce exist in every sea, but certain groups are more characteristic of cold and 
temperate climates. Bela, for instance, may be regarded as a representative 
northern genus; whereas, on the other hand, Cythara and Pusionella are tropical. 
They are generally rare in individuals, although numerous in species. They occur 
at all depths, from low-water mark to two thousand or more fathoms; indeed, a 
very delicate form, about an inch in length ,—Clathurella monoceros ,—was dredged 
in mid-Atlantic, to the south-west of Sierra Leone, in two thousand five hundred 
fathoms. The extinct forms of this family, occurring chiefty in the Tertiary 
deposits, are perhaps as numerous as those now living. 
In the family Cancellariidce the proboscis is small or rudimentary, the foot 
small, the siphon very short, while the eyes are placed at the outer base of the 
tentacles; the operculum being wanting. The Cancellarias have very beautiful 
shells, generally elegantly sculptured with longitudinal and spiral ridges, producing 
a cancellated surface. The forms are variable; some are oval, others turreted, and 
the most remarkable of all (Cancellaria trigonostoma ) has the spire drawn out 
like a cork-screw. A constant feature in this family consists of three or four folds, 
plaits, or plicae upon the columella. A few of the species are common, but many 
are extremely rare. About a hundred different forms are known. They are 
mostly tropical, but the genus Admete has a more northern range, extending as 
far as Greenland. They are usually obtained in shallow water, but a few specimens 
have been dredged at a depth of nearly seven hundred fathoms. 
SECTION RHACHIGLOSSA. 
This group of Pectinibranchs includes a large number of genera and species 
superficially very different. They are all marine gastropods, with a well-developed 
extensile proboscis, sometimes as long as, or even longer than the body. The 
radula is generally long, narrow, and armed with three teeth in a transverse row, 
