XIV MONOTOCARDIA — TAENIOGLOSSA 415 



Fam. 1 6. Eissoidac. — Eyes at the external base of the 

 tentacles, epipodium with filaments, operculigerous 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 water. Jurassic . Principal genera : 



Bissoa (subg., Folinia, Onoha, Alvania, Cingula, Nodulus, Anaba- 

 thron, Fenella, Iravadia, and others), Sccdiola (shell agglutinating 

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

 an apophysis- as in Nerita), Barleeia, Faryphostoma (Eocene). 



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

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

 gerous 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); Pomatiopsis, Hydrohia, Bithynclla, 

 Micropyrgus (Tertiary), Fyrgula, Fmmericia, Bcnedictia, Litho- 

 glyplius, Tanganyicia, Limnotrochus (?), Jullienia, PachydroMa, 

 Fotamopyrgus, Littorinida, Amnicola, Flurninicola (subg., Gillia, 

 Somatogyrus), 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. Skencidae.—-Jxadn\a resembling that of Hydrohia, 

 shell very small, depressed, widely umbilicated, operculum 

 corneous. Pleistocene — . Single genus, Skenea. 



Fam. 20. Jeffrey siidae. — 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 i-ather swollen, operculum with 

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

 Genera : Jeffreysia, Dardania. Marine, living on algae. 



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

 operculigerous 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, Bicda. 



