LTMN^ID^. lOT 



depressed, whorls numerons, carinated ; apertvire oblique. A. 

 keronatdid es, d^Orh. (ciii, 33). 



DREPANOTREMA, Crosse and Fischer. Whorls subglobose, the 

 last enveloping, aperture narrowly lunate. Central America. 

 P. Ysabalensis. 



Segmenttna, Fleming, 1830. 



Sy7i. — ^Discus, Hald., 1840. Segmentaria, Swains., 1840. 



Distr. — Europe, Asia, Australia. S. lacustris, Lightf. (ciii, 34). 

 Europe. 



Shell flattened ; whorls usually laterally compressed, with 

 angulated periphery, the last whorl embracing, internally 

 contracted by periodic lamellae, usually three in number, and 

 occurring three times in each volution. 



PLANORBULA, Hald., 1840. Shell discoidal, with rounded 

 whorls, divided interiorly at intervals by septa consisting of five 

 lamelliform teeth. S. armigera, Say (ciii, 35, 36). United 

 States. 



Subfamily ANGYLIN^. 



Shell non-spiral, patelliform or conical. 



Ancylus, GeofFroy, 1767. 



Etym. — Ancylus^ a small round shield. River limpet. 



Syn. — Angulus, Miihlfeldt. 



Distr. — 50 sp. N"orth and South America, Europe, Australia. 

 Fossil, 8 sp. Eocene, Europe ; Laramie — , United States. 

 A. concentricus, d'Orb. (ciii, 37). 



Shell conical, limpet-shaped, thin ; apex posterior, turned to 

 the left ; aperture with entire, basal margin ; interior with a sub- 

 spiral muscular scar. 



Animal with large oval foot ; tentacles triangular, with eyes 

 at their internal bases; jaw thin; central tooth very small, 

 laterals bicuspidate, marginals saw-like. 



ancylastrum, Moquin-Tandon, 1853. Genital and pulmonary 

 orifices on the left side. Apex of the shell inclining to the right. 

 A. JiuviatiUs^ Miiller. 



ACROLOXUS, Beck, 1837. (Yelletia, Gray, 1840.) Apex sinistral ; 

 shell narrow, oblong. Animal with genital and pulmonary 

 openings on the right side. A. lacustris^ Linn, (ciii, 38). 3 sp. 

 Europe, West Ladies, United States. 



CUMINGIA, Clessin. Shell conical, top coiled, aperture oval. 

 A. Cumingianus, Clessin. 



HALDEMANiA, Clcssin. Shell conical, top slightly excentric, 

 but not bent backwards, aperture rounded or oval. A. obscuruS, 

 Hald. United States. 



LANX, Clessin. Shell large, patelliform, elevated at the extrem- 

 ities, thin, with concentric striae ; apex very obtuse, rounded ; 

 aperture ovate, the margin acute. A. Neivberryi, Lea. 



