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 sjDecies 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. 'pacjo- 

 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 chiefly 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 (C<i n cellar ia irigonostoma) 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 Adraete 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 EHACHIGLOSSA. 



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, 



