184 PROCEEDINGS OF THE ACADEMY OF 
Genus HELICODISCUS, Morse. 
Jaw according to Morse, of the only known species, H. lineatus, 
low, wide, cresentic, ends much attenuated, acute ; cutting margin 
with a median, beak-like projection ; 
anterior surface without ribs, but 
EZ : : 0 
LEE» = covered with strize converging ob- 
<i liquely towards the beak-like promi- 
nence. 
Jaw of H. lineatus. [Morse.] 
According to my rule of admitting in the genus Helix only such 
species as have a ribbed jaw, I am forced to recognize lineatus as 
a distinct genus. Fig. 27 shows the general arrangement of the 
Fig. 27. 
7 ingual dentition of H. lineatus. ieee J 
teeth upon the lingual membrane. The characters of the separate 
teeth are better shown in my plate XIII. fig. 5. Morse gives 17 
rows of 12—-1—12 teeth, each with 4 perfect laterals. Leidy, in 
Terr. Moll. U.S., II. 262, fig., gives 13—1—13 teeth, with 5 perfect 
laterals. The membrane examined by me has 12—1—12 teeth, 
with 4 perfect laterals. The central teeth have a base of attach- 
ment very small, longer than wide, with expanded lower angles, 
and reflected upper margin. Reflection very small, with a stout, 
short, median cusp, and very short, blunt side cusps, all the 
cusps with short cutting points. The lateral teeth have a base of 
attachment three times as wide, and somewhat longer than the 
centrals, and unsymmetrical by the suppression of the inner, lower 
lateral expansion; the upper margin is broadly reflected; the 
reflection is short but symmetrical, having two equally developed 
short, stout side cusps, bearing short cutting points; the median 
cusp is stout, long, extending nearly to the lower edge of the 
base of attachment, Beyong which projects slightly the short 
cutting point. 
The marginals are low and wide, the reflection as broad as the 
base of attachment, reaching nearly to its lower edge, and furnished 
