41 DIOTOCARDIA— RHIPIDOGLOSSA chap. 



Fam. 8. Maeluritidae. — Shell discoidal, whorls few, longi- 

 tudinally grooved behind, right side convex, deeply umbilicated, 

 left side flat ; operculum very thick, nucleus excentrical, internal 

 face with two apophyses, one very large. The general appearance 

 is more that of an inequivalve bivalve, such as Requienia^ than of 

 a spiral gasteropod. Palaeozoic . Single genus, Maelurea. 



Fam. 9. Neritidae. — Snout short, tentacles long, eyes pedun- 

 culate at their outer base, branchia triangular, free at the front 

 end, epipodium without cirrhi, penis near the right tentacle; 

 shell solid, imperforate, turbinate to almost patelliform, spire 

 short, internal partitions absorbed (p. 168), columellar region 

 broad, edge simple or dentate, operculum calcareous, spiral or 

 non-spiral, with prominent apophyses on the interior face, one 

 of which locks behind the columellar lip. Jurassic . Prin- 

 cipal genera : Nerita (Fig. 13, p. 17) ; Neritina (chiefly brackish 

 water and fluviatile), sub-genus Olithon, usually coronated with 

 spines ; Velates (Tertiary), Neritoma (Jurassic), Beianira (Cre- 

 taceous), Septaria (^ = Navicella)^ shell more or less narrowly 

 patelliform, with terminal apex, aperture very large, with a 

 broad columellar septum, operculum too small for the aperture, 

 more or less covered by the integument of the foot ; fluviatile 

 only; Pileolus (Jurassic to Cretaceous). 



Fam. 10. Hydrocenidae. — Branchia replaced by a pulmonary 

 chamber, eyes at the outer base of the tentacles, marginals of 

 the radula very oblique, centrals often wanting; shell small, 

 conical, whorls convex, operculum calcareous, with a prominent 

 apophysis. Recent. Principal genera : Hydrocena^ Georissa. 



Fam. 11. Helicinidae. — Branchia replaced by a pulmonary 

 chamber, heart with one auricle ; shell globular, with a short 

 spire, internal partitions absorbed; operculum without apophysis. 



Carboniferous . Principal genera: Helicina (Fig. 18b, p. 



21; subg. Alcadia^ Sehasicheila^ Heudeia^ Calyhiuni)^ Eutrocha- 

 tella (subg. Lucidelld)^ Stoastoma, Boiireieria, Bawsonella (Car- 

 boniferous). 



Fam. 12. Proserpinidae. — Branchia replaced by a pulmonary 

 chamber, mantle partly reflected over the shell, eyes sessile ; 

 shell depressed, discoidal, columella folded or truncated at the 

 base, whorls with one or more internal plicae, internal partitions 

 absorbed, no operculum. Eocene . Single genus; Proser- 

 pina^ subg. Proserpinella^ Cyane^ Bimorplioptycliia (Eocene), 

 and Ceres (Fig. 18c, p. 21). 



