FAMILY PLEUIWTOMW^. 



Shell fusiform, with a more or less produced anterior canal, and 

 a slit or sinus of the outer margin of the aperture near the suture. 

 Operculum (not always present) corneous, annular, the nucleus 

 apical, or subcentral and nearly marginal. 



Animal with widely separated tentacles, the eyes usually at or 

 near their base ; mantle generally with a sinus on the right margin 

 corresponding with the sinus of the shell ; siphon long. Denti- 

 tion : usually there are no central teeth, and the laterals are a 

 single one on either side of the lingual band (1-0-1) ; but in 

 some groups there is a central tooth, and in others there are 

 two laterals. No jaws. 



The dentition, however it varies in minor respects, always pre- 

 serves a resemblance to that of the Conidse, Terebridre and Can- 

 cellariidae sufficient to include it with these in a great group 

 Toxoglossa. The teeth are long, usually subulate, supplied with 

 venom from a large gland (PI. 33, fig. 52). 



There is some resemblance in the sinus of the shell between 

 Conus and the principal groups of Pleurotoniida3 ; and even in 

 form, Genotia is connected, through Conorbis, with Conus. On 

 the other hand Pusionella seems to form the connecting link 

 with Terebra, Halia with Cancellaria, etc. 



In no other group of mollusks is it so difficult to make a satis- 

 factory classification as in the Pleurotomidae. The forms are 

 exceedingly numerous, and known in many species to be very 

 variable in their characters, whilst the material for the recognition 

 of most of those described is generally scant} 7 . Of the figured 

 species, a very large proportion were described from single or 

 few specimens, and most cabinets, however large, do not possess 

 shells which can be certainly identified with these : then there is 

 an unusually large proportion (amounting to hundreds) of un- 

 figured species, the recognition of which is simply impossible. 



The many generic and subgeneric groups that have been made, 



far from enabling us to arrange the species in something like 



systematic order, only increase the confusion ; for so great is the 



variability of all the characters that nearly allied species have 



11 ' (151) 



