XIV MONOTOCARDIA— TAENIOGLOSSA 415 



Fam. 16. Rissoidae. — Eyes at the external base of the 

 tentacles, epipodium with filaments, opercnligerous lobe with 

 appendages ; central tooth pleated at the basal angles, lateral 

 large, bluntly multicuspid, marginals long, narrow, denticulate 

 at the edge ; shell small, acuminate, often elaborately sculptured, 

 mouth entire or with a shallow canal, operculum corneous. 



Marine or brackish w^ater. Jurassic . Principal genera: 



Rissoa (subg., Folinia^ Onoha, Alvania, Cingula^ Nodulus, Anaha- 

 thron, Fenella^ Iravadia^ and others), Scaliola (shell agglutinating 

 fragments of sand, etc.), Rissoina (lip thickened, operculum with 

 an apophysis as in JSferita)^ Barleeia^ Parypliostoma (Eocene). 



Fam. 17. Hydrohiidae. — Eyes at the outer base of the 

 tentacles, penis behind the right tentacle, prominent, opercnliger- 

 ous lobe without filaments ; radula rissoidan, central tooth often 

 with basal denticulations ; shell more or less acuminate, small, 

 aperture entire, operculum corneous or calcareous. Brackish or 



fresh water. Jurassic . Principal genera : Baicalia, with 



its various sub-genera (p. 290); JPomatiopsis, Hydrohia^ Bithy- 

 nella, Micropyrgus (Tertiary), Pyrgula^ Emmericia^ Benedietia, 

 Jjithoglyphus^ Tang any icia^ Limnotrochus (?), Jidlienia^ Pachy- 

 drohia, Potamopyrgiis, Littorinida, Amnicola, Fluminicola (subg., 

 G-illia^ Somatogyru8)^Bithynia^ Fossarulus (Tertiary), Stenothyra. 



Fam. 18. Assimineidae. — Ctenidium replaced by a pulmonary 

 sac, no true tentacles, eye-peduncles long, retractile ; radula that 

 of Hydrohia; shell small, conoidal, operculum corneous, nucleus 

 sub-lateral. Eocene . Genera : Assiminea^ Acmella. 



Fam. 19. Skeneidae. — Radula resembling that of Hydrohia ; 

 shell very small, depressed, widely umbilicated, operculum 

 corneous. Pleistocene . Single genus, Skenea. 



Fam. 20. Jeffreysiidae. — Mantle with two pointed ciliated 

 appendages in front, tentacles ciliated, eyes sessile, far behind 

 the base of the tentacles; marginal teeth sometimes absent; 

 shell small, thin, pellucid, whorls rather swollen, operculum with 

 marginal nucleus, divided by a rib on the inner face. Recent. 

 Genera: Jeffreysia^ Bardania. Marine, living on algae. 



Fam. 21. Litiopidae. — Epipodium with cirrhi on each side, 

 opercnligerous lobe with appendages ; radula rissoidan ; shell 

 small, conical, columella truncated, operculum corneous. Eocene 



. Genera : Litiopa^ living on the Sargasso weed, suspended 



by a long filament ; Alaha^ Diala. 



