Rissoa.] GASTROPODA. 199 



Genus 1. RISSOA, Freminville, 1814. 



Rissoa, Frem., Nouv. Bull. Soc. Ph., vii, 1814. Type : P. ventricosa, Desmarest. 



Animal having long, slender tentacles, sometimes ciliated, with 

 eyes on small prominences near their outer bases ; mantle with a 

 tentacular appendage in front, on the right side, or on either side ; 

 foot truncate in front, attenuated behind, with a glandular sulcus on 

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

 either side, with 1 to 3 posterior filaments. Radula with the central 

 tooth subquadrangular, multicuspidate, base broadened, incised and 

 lobed laterally ; lateral and marginal teeth multicuspidate. 



Shell imperforate or subperforate, more or less oblong, turbinated, 

 usually thick, white or corneous, smooth, libbed or cancellated ; aper- 

 ture entire, oval ; peristome thick, simple or reflected ; operculum 

 corneous, paucispiral. 



Over 400 species have been made known to science. From their 

 minute size it is certain that a good many are synonyms ; but, on the 

 other hand, it is probable that many more remain to be described. 



They inhabit all seas, from high water to about 1,100 fathoms, 

 some species, therefore, belonging to the abysmal fauna. They abound 

 most in shallow water, near shore, on beds of seaweed. 



The animal is active and bold, floats about, and spins a byssal 

 thread instantly upon being detached from its foothold. The in- 

 cessant play of the tentacular cilice is very striking. The epipodial 

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



There are a few Secondary species, but the genus is principally 

 found fossil in Tertiary beds. 



Vernacular Name. Spire-shell. 



KEY TO SUBGENERA. 



A. Shell ribbed i.e., the striking features are axial ribs . . RISSOA, s.s. 



B. Shell reticulated i.e., ribs and spirals pretty equal in strength . . ALVANIA. 



C. Shell strongly spirally lirate i.e., the spiral threads being the 



striking feature . . . . . . . . . . ONOBA. 



D. Shell weakly spirally lirate i.e., there are spiral threads, but 



they are not strong . . . . . . . . . . CERATIA. 



E. Shell with flat and smooth whorls . . . . CINGULA. 



F. Shell with rounded and smooth whorls . . . . - - SETIA. 



Subgen. 1. RISSOA, s. str. 



Shell elongated conic, axially costate ; aperture oblong, with a 

 thickened peristome bearing an exterior rib. 



KEY TO SPECIES. 



A. Shell with spiral sculpture. 



a. A spiral groove belowthe suture, costae extending over the base impressa. 

 aa. A spiral thread on the periphery of the body- whorl, stopping 



the axial costse . . . . . . . . . . Hamilton*. 



B. Shell without spiral sculpture. 



a. Shell oval, riblets obsolete on the base, spire twice the height 



of the aperture . . . . . . . . . . Huttoni. 



aa. Shell elongate, riblets extending over the base, spire 2| 



times the height of the aperture. . . . . . . rufoapicata. 



