106 



LAND AND FRESH-WATER SHELLS OF N. A. [PART I. 



forms a beautiful variety, and has been thought to be a distinct 

 species. 



The form known as H. volvoxis is found on the Atlantic coast 

 of Florida and Georgia. It is thus described by Pfeiffer. The 

 synonymy is also given in full. I believe it to be a variety of 

 H. sejytemvolva : — 



Shell umbilicated, orbicularly convex, thin, reddish horn-colored, pel- 

 lucid, with regular rib-like striae ; spire very short, convex ; whirls seven, 

 convex, regularly increasing, the last larger above than the rest, angular, 

 below the angle inflated, striated and shining ; iimbilicus large, regular, 

 in which the whirls regularly decrease, excepting the last, which is very 

 broad ; aperture rather large, kidney-shaped ; peristome thickened within, 

 reflected, its terminations joined by a short, triangular, tooth-like callus. 

 Greater diam. 9, lesser 8 ; height 4 mill. 



Helix volvoxis, Pakkeyss in Pfeiffer, Symb, III, 80 ; Mon. Hel. Viv. I, 

 409 ; in Chemnitz, ed. 2, I, 379 (184G), pi. Ixvi, f. 4-6 (1849).— 

 Reeve, Con. Icon. no. 1237 (1854).— \V. G. Binney, Terr. Moll. U. S. 

 IV, 92, pi. Ixxviii, f. 17.— Bland, Ann. N. Y. Lye. VII, 135. 



Polygyra volvoxis, Tryon, Am. Journ. Conch. Ill, 159, pi. xi, f. 25 (1867). 



Fig. 182. 



IIeli:x cereolMS, Muhlfeldt. — Shell broadly umbilicated, subcari- 

 nated, discoidal, white, scarcely convex and with rib-like strife above, 

 smooth and plane below ; whirls seven or eight, 

 gradually increasing, the last subcarinated, briefly 

 deflected at the aperture, constricted behind the 

 peristome ; below three full whirls revolving on 

 the same plane, the balance visible in the broad, 

 pervious umbilicus, the penultimate somewhat 

 lapped over by the last, the antepenultimate the 

 most swollen ; aperture remote from the axis, sub- 

 reniform ; peristome white, thickened, acutely re- 

 flected, somewhat angular at the carination of the 

 last whirl, continuous, its terminations joined by 

 triangular, elevated, acutely-pointed callus ; on 

 the parietal side of the inner fourth of the last> 

 and running round rather obliquely within from 

 two-thirds to three-fourths of the penultimate 

 whirl, thus revolving nearly once round the shell, 

 is a thread-like, elevated, white internal lamina. 

 Greater diam. 14, lesser 12} ; height 3} mill. 



JJelix eereolus, enlarged. 



