LIMN HIDAL 107 
depressed, whorls numerous, carinated; aperture oblique. A. 
kermatoides, d’Orb. (ciii, 33). 
DREPANOTREMA, AS and Fischer. Whorls subglobose, the 
last enveloping, ‘aperture narrowly lunate. Central America. 
P. Ysabalensis. 
SEGMENTINA, Fleming, 1830. 
Syn.—Discus, Hald., 1840. Segmentaria, Swains., 1840. 
Distr. —Europe, Asia, Australia. S. lacustris, Light?. (cili, 34). 
Europe. 
Shell flattened; whorls usually laterally compressed, with 
angulated periphery, the last whorl embracing, internally 
contracted by periodic lamelle, 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. 
Supramity ANCYLINA. 
Shell non-spiral, patelliform or conical. 
Ancytus, Geoffroy, 1767. 
Etym.—Ancylus, a small round shield. River limpet. 
Syn.—Angulus, Muhlfeldt. 
Distr.—50 sp. North 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 sear. 
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. fluviatilis, Muller. 
ACROLOXUS, Beck, 1887. (Velletia, 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 Indies, United States. 
cumiINGtIA, Clessin. Shell conical, top coiled, aperture oval. 
A. Cumingianus, Clessin. 
HALDEMANIA, Clessin. 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 strize; apex very obtuse, rounded ; 
aperture ovate, the margin acute. A. Newberryi, Lea. 
