GASTEEOfODA. 303 



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

 quadrate, central and marginal bicuspid, laterals tricuspid ; 

 excretory orifices on left side of the neck. 



Some species of Planorhis haye the sutures andsj^ire deeply sunk, 

 and the umbilicus flattened ; sj)ecimens occur with 

 the spire elevated (Fig. 130*). P contortus, 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) Fig. 130. 

 has 5 teeth in the apertiu-e which nearly close the passage. 



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

 China. 



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



Guncllachia, ancyliformis, Pfeiffer, 1850. Fresh waters. Cuba. 



Shell thin, obliquely conic; apex inclined i)osteriorly ; base 

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

 circular. 



Family Y. — AinncuLiDiE. 



Shell spiral, covered with horny epidermis, spire short, body- 

 whorl large ; aperture 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; I, laterals. 



^iiil^^^^^:^::^^'^ ^—^- 



^;;^^< ^i^?§^sS2^^ 



::>«;; 



"'™>««<3!!!!i::r'"""'™~™'»~ 



Fig, 131. 



The Atiriculce 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 

 Tomatella and liingicula. 



* P. margtnaius, var. Koclitlale, communicated by J. S. Gaekoin, Esq. 



