28 HELICIDA. 
at the periphery. N. conoidalis, Ad. and Reeve (xciii, 84). 
Philippines. 
THAPSIA, Albers, 1860. Shell orbicularly depressed, thin, pel- 
lucid, undulately decussated, narrowly perforated ; whorls six, 
slowly increasing ; peristome simple, acute; columellar margin 
narrowly reflected. N. troglodytes, Morelet. 
Famity HELICID &. 
Shell spiral, usually thicker than in the Zonitidee, and mostly 
with reflected lip, the aperture edentulous or contracted by teeth. 
Animal capable of complete retraction within the shell; the 
jaw finely striate, or ribbed, sulcate or plicate; teeth, central 
tricuspid, laterals bicuspid or tricuspid with an obsolete internal 
cusp, marginals usually wider than high, short, with two or 
three small cusps (xiii, 59). 
HeEix, Linnaeus. 
Distr.—3400 sp. Universal. Fossil; Cretaceous—. Europe; 
Laramie—. U.S. 
Shell of variable form, smooth, rugose, striate, ribbed or 
tuberculate, sometimes pilose; orbicular convex, planorboid, 
trochiform, subturriculated, or short bulimiform (monstrosities 
sinistral, or with the whorls more or less uncoiled); aperture 
oblique, oval, or semilunar, with or without interior teeth on 
the margin or-parietal wall; lip simple or thickened internally or 
reflected ; umbilicus covered to widely open. 
No precise diagnosis can be given of a genus in which the 
characters of the shell vary so much as in Helix. Albers, Beck, 
Swainson, Ferussac, H. and A. Adams, etc., have proposed a 
great number of groups, the species of which possess usually the 
double value of similar characters coinciding with similar distri- 
bution. Thus the species of a subgenus or section of Helix 
very generally strongly suggest by their facies and territory 
their descent from a common ancestry. 
The number of species of Helix, although reduced by the 
elimination of the genera Nanina and Zonites, is still so large 
that a further separation would be very desirable; such groups 
as Patula, Sagda, etc., could be used in a generic sense with 
great advantage, provided conchologists would cease to apply 
to them the familiar designation Helix. 
sacDA, Beck, 1837. (Hpistylia, Swainson, 1840.) Shell not 
umbilicated, globosely conoidal; spire more or less elevated, 
with obtuse apex; eight or nine whorls, the last flattened at the 
base, excavated around the umbilical region, with internal 
revolving lamelle ; columella short, oblique, dilated at the base ; 
aperture obliquely semilunar; peristome simple. Jaw oxygna- 
thous. 13 sp. Jamaica. H. alligans, Ads. (xciil, 2). 
