3342 THE MOLLUSKS OR SHELLFISH 



Beta, 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 

 southwest of Sierra L,eone, 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 Cancellariida the proboscis is small or rudimentary, the foot 

 small, the siphon very short, while the eyes are placed at the outer base of the ten- 

 tacles; 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 

 corkscrew. 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, 

 one central or rhachidian, and one lateral on each side; the latter, however, are 



wanting in certain groups. 

 The respiratory siphon is 

 rather long, fitting into 

 the anterior canal or notch 

 of the aperture. The eggs 

 of these mollusks are de- 

 posited in strong leathery 

 capsules, each capsule 

 often containing a large 

 number of ova. The 



BI.ACK owvs, Oliva maura. S enus Yetus > however, 



(Natural size.) which is viviparous, pro- 



d u c e s a comparatively 



small number of young. All have shells, which in nearly all cases are capable of 

 sheltering the entire animal. The principal families in this group are the Olividtz, 

 Harpidce, Marginellid<z , Volutidce, Mitridce, Fasciolariidce > Turbinellidce, Buctinid<z y 

 Nassidce, Columbellidce , Muricidce, and Corolliophilidce. 





