GASTEEOroDA. 303 



slender, the eyes at their inner bases ; lingual tc-etli sub- 

 quadrate, central and marginal bicuspid, laterals tricuspid ; 

 excretory orifices on left side of the neck. 



Some species of Planorhis hare the sutures and spire deeply sunk, 

 and the umbilicus flattened ; specimens occur with 

 the spire eleyated (Fig. 130*). P contortiis, a minute 

 species, has above 6,000 teeth. (Cocken.) P. corneus 

 secretes a purple fluid. (Lister.) P. lacustris (Seg- 

 mentina, Fleming) has the "whorls contracted inter- 

 nally by periodic septa, 3 in a whorl, with triradiate 

 openings. P. armigenus (Planorbula, Haldeman) ^'g- 1^*^- 

 has teeth in the aperture which nearly close the passage. 



Distrihution, 145 species. North America, Europe, India; 

 China. 



Fossil^ 69 species. "Wealden— . Britain, France. 



Gundlachia, ancyliformis, Pfeifi'er, 1850. Fresh waters. Cuba. 



Shell thin, obliquely conic; apex inclined posteriorly; base 

 closed for two-thirds by a flat, horizontal plate ; aperture semi- 

 circular. 



Family Y. — Atjeictilid^. 



Shell spiral, coyered with homy epidermis, spire short, body- 

 whorl large ; apertui-e elongated, denticulated ; internal septum 

 progressively absorbed. 



Animal with a broad and short muzzle, tentacles, 2, cylin- 

 drical, the eyes sessile behind them ; mantle-margin thickened ; 

 orifices as in the snails ; foot oblong ; sexes united ; mouth 

 with a horny upper jaw ; lingual teeth numerous, central series 

 distinct, hooked, tricuspid. A. livida has about 31 laterals 

 (Loven) ; another species examined by Mr. Wilton has 11 

 large laterals and about 100 smaller (imcini) on each side, 

 gradually diminishing towards the edge (Fig. 131) : c, central 

 teeth; ', laterals. 



^^a 



Fig. 131. 



The Auricula', frequent salt-marshes, damp hollows, and 

 places overflowed by the sea; they were long regarded as 

 marine animals, and their shells confused with those of 

 Tornatella and Ringicula. 



* P. marginatus, var. Rochdale, communicated bj- J. S. Gaskoin, Esq. 



