AIO DIOTOCARDIA RHIPIDOGLOSSA chap. 



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

 tudinally grooved behind, right side convex, deeply unibilicated, 

 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 Eequienia, than of 

 a spiral gasteropod. Palaeozoic . Single genus, Maclurea. 



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

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

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

 shell solid, imperforate, turbinate to almost patellifoiin, 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 . Principal genera : 



Nerita (Fig. 13, p. 17); Neritina (chiefly brackish water and 

 fluviatile), sub - genus Clithon, usually coronated with spines ; 

 Velates (Tertiary), Neritoma (Jurassic), Deianira (Cretaceous), 

 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 ; Pilrolus 

 Jurassic to Cretaceous). 



Fam. 10. Hydroceriidac. — 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, witli a prominent 

 apophysis. Recent. Principal genera ; Hydrocena, Georissa. 



Fam. 11. IMicinidae. — 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. 18, B, p. 



2 1 ; subg. Alcadia, Schasicheila, Heudeia, Cah/hium), Eutrochatclla 

 (subg. Lucidella), Stoastoma, Bourcieria, Dawsonella (Carboniferous). 



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. Proseiyinelln, Cyane, Diinorphoptychia (Eocene), and 

 Ceres (Fig. 18, C, p. 21). 



