GASTEROPODA. 



Shell turriculated, thick ; aperture circular, continuous, v.'ith 

 a large expanded border. 



Fossil, 2 species. Saint Cassian beds, Austria. 



HoLOPELLA, McCoy, 1852. 



Example, H. gregaria, Sow. (Turritella), Sil. Syst. t. 3, f. 1. 



Etymology, 'olos, entire, and ope, an aperture. 



Shell elongated, slender, of numerous gradually increasing 

 whorls, generally crossed by slightly arched striae ; mouth cir- 

 cular, with the peristome entire ; base rounded, with or without 

 a minute umbilicus. 



The shells of the species composing this genus differ from 

 those of . Turritella in the continuous peristome and definite 

 round margin to the aperture, thus approaching much nearer 

 to Seal aria. 



Fossil, 12 species. Silurian — Trias. Europe, United States. 



Family IY. — Melajstiad-^.* 



Melajstia. — Tentacles long, with eyes on the exterior side at 

 about a third of the length ; margin of the niantle festooned. 



Sub-genera, Vihex, Melanatria, Hemsinus, and 



Philopotamis, Layard, P. sulcata, Eeeye, sp. Operculum sub- 

 spiral ; nucleus marginal. Shell solid, paludiniform. Distribu- 

 tion, 5 species. Ceylon. Habit of Tanalia. 



Paxudomus {Type, P. conicus. Gray), as restricted by the 

 separation of Philopotamis and Tanalia, is characterised by the 

 concentric structure of the adult operculum resembling that of 

 Paludina, and a spiral nucleus situated about the middle of its 

 height, and nearest to the left margin. 



Distribution, India, Burmah, Egypt, East Indian Archipelago, 

 Mauritius, Ceylon (2 species, reduced from 14). In tanks and 

 marshes. 



Sub-genus, Tanalia, Gray. 



Synonym, Ganga, Layard, founded upon certain monstrous 

 forms of T. aculeata. 



Type, T. aculeata Chemnitz. 



Shell semiglobose, costate, nodulose; mouth very large, OYate; 

 operculum unguiculate ; nucleus marginal. 



Distribution, 2 species. Inhabiting mountain streams, ad- 

 hering to rocks, or crawling over sandy bottoms, Ceylon. 



Fossil, 2 species. Upper Chalk. Gosau 



* See p. a46, &c. 



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