BISSOIIDiE. 'iir) 



ou its lower side ; operculigerous lobe with a wing-like expansion 

 on either side, with one to three posterior filaments. Jaws oval, 

 lingual teeth 2-1-1-1-2 (PI. 54, fig. 98). 



Fully 200 recognized species have been made known to science, 

 besides unfigured species and synonyms amounting to perhaps 400 

 more. From their minute size it is probable that many more re- 

 main to be described. They inhabit all seas, from high water to 

 100 ththoms, but being phytophagous, abound most in shallow 

 AViatcr, near shore, on beds of seaweed. 



There are a few sec(mdary species, but Rissoia is principally 

 found fossil in tertiary beds. 



Rissoia is active and bold, floats like its congeners, and spins a 

 byssal thread instantly u}>()n being detached from its foothold. The 

 incessant play of the tentacular ciliaj is very striking. The pallial 

 filaments probably are, like the tentacles, tactile in function. 



Subgenus Rissoia (sensu stricto). 



Shell elongated conic, longitudinally costate ; ajjcrture oblong, 

 Avith a thickened peristome bearing an exterior rib. 



R. VENTRIC08A, Desmarest. 



The characters of the following subordinate groups so merge one 

 into another that in many cases the relegation of sjiecies to a par- 

 ticular division is an entirely arbitrary matter. I shall indicate 

 the sections ; but it must be understood that many of the species 

 have not been assigned heretofore to any section, and that others 

 belong to two or more sections, according to the variation of their 

 characters. No doubt the institution of divisions founded upon 

 such unsubstantial characters has tended to the undue multiplica- 

 tion of species ; but I am unable to satisfactorily arrange the sy- 

 nonymy — this can only be done by one who will devote himself to 

 the subject, as a specialty. 



Section Apicularia, Mouterosato. bS84. 



Shell turbinate, whorls and aperture more rovuided, the inter- 

 stices of the rounded ribs spirally striate. R. Ehrenbergi, Phil. 



Section Rissostomia, Sars. 1878. 



Shell very thin, smooth or slightly longitudinally costate, often 

 with zigzag longitudinal fulvous markings. 



R. MEMBRANACEA, Adams. 



