28 HELICID^. 



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. 



Family HELICID^. 



Shell spiral, usually thicker than in the Zonitidse, 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). 



Helix, Linnaeus. 



Bistr. — 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 ISTanina 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. 



SAGDA, Beck, 1837. (Epistjdia, 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 nmbilical region, with internal 

 revolving lamellae ; columella short, oblique, dilated at the base ; 

 aperture obliquely semilunar ; peristome simple. Jaw oxygna- 

 thous. 13 sp. Jamaica. H. alligans, Ads. (xciii, 2). 



