HELICODISCUS. 185 



■with a stout, short, median rnsp, and vor}' short, hlunt side cusps, all the cusps 

 with short cuttin<j^ points. The lat«'ral teeth have a base of attachment three 

 limes as wide, and somewhat longer than the centrals, and asyinmetri(;al by 

 the suppression of the inner, lower lateral expansion; the upper marj^in is 

 broadly reflected ; the reflection is short but symmetrical, having two equally 

 developed short, stout side cusps, bearing short cutting })oints ; th(! median 

 cusp is stout, long, extending nearly to the lower edge; of the base of attach- 

 ment, beyond which projects slightly the short cutting point. 



The marginals are low and wide, the reflection as broad as the base of at- 

 tachment, reaching nearly to its lower edge, and furnished with one inner, 

 long, bluntly bifid, stout, oblique cutting point, and two or more short outer cut- 

 ting points. The same form of marginal is found in Pupa. 



The membrane is very peculiar in the lateral teeth, not only from their large 

 size, but also from their symmetrical, tricuspid reflection, quite like the usual 

 arrangement of central teeth in the Helicea. Similar lateral teeth are found 

 in Zonites Gundlachi. 



Helicodiscus lineatus, Say. 

 Vol. III. PI. XLVIIL Fig. 1. 



Shell widely umbilicated, discoidal ; epidermis greenish ; whorls about 4, 

 visible on the base of the shell as well as above, with numerous equidistant, 

 parallel, raised lines revolving upon them ; suture much impressed ; aperture 

 remote from the axis, semi-lunate, narrow, not expanding ; peristome acute, 

 thin ; umbilicus wide, forming a concave depression of the base, each volution 

 visible to the apex ; within the aperture, on the external circumference, are 

 placed from 1 to 3 pairs of minute, conical, white teeth, the first pair in sight 

 when looking into the aperture, the others more remote. Greater diameter 3^, 

 lesser 3 mill.; height, 1^ mill. 



Helix liiuata, Say, Journ. Phila. Acad., I. 18 (1817) ; II. 273 (1824) ; Nich, 

 Encycl., 3d ed., IV. (1819) ; Binney's ed. 7, 24. — Binney, Bost. Journ. Nat. 

 Hist., III. 436, PI. XXII. Fig. 6(1840); Terr. Moll., II. 261, PI. XLVIII. 

 Fig. 1. — DeKay, N. Y. Moll., 44 (1843). — Gould, Invert, 179, Fig. 103 

 (1841). —Adams, Vermont Mollusca, 161 (1842). — FitiiussAC, Tab. Syst, 44 ; 

 Hist., PI. LXXIX. Fig. 1. — Deshayes in Fer., I. 80. — Chemnitz, 2d ed., 

 II. 203, tab. CI. Figs. 13 - 15. — Pfeiffer, Mon. Hel. Viv., I. 184. — Reeve, 

 Con. Icon., 724 (1852). —W. G. Binney, Terr. Moll., IV. 123. — Morse, 

 Amer. Nat., I. 546, Fig. 44 (1867). 



Planorbis parallelus, Say (?), Proc. Acad. Nat. Sci., II. 164 (1821); ed. Binney, 

 63. 



Hyalina ? liiuata, W. G. Binney, L. & Fr.-W. Sh., I. 52 (1869). —Gould and 

 Binney, Invert, of Mass., ed. 2, p. 404 (1870). 



Helicodiscus liiieata, Morse, Journ. Portl. Soc., I. 25, Figs. 61, 62, PI. II. Fig. 

 3; PI. VIII. Fig. 63 (1864). — Tryon, Am. Journ. Conch., II. 264 (1866). 



