LYMNIDA OF NORTH AMERICA. 121 
Nervous system (including sense organs such as tentacles, etc.). 
Muscle system. 
While the Stylommatophorous and Basommatophorous pulmon- 
ates differ rather widely in details, it would seem that the above criteria 
apply with equal force in formulating a natural classification of the 
fresh-water pulmonates and it has accordingly been followed as far 
as practicable. In the treatment of the genera of the Lymnezas the 
shell, radula, jaw and genitalia have been considered of principal im- 
portance and have been comparatively studied. In the case of the sub- 
genera, the shell and radula have been relied upon to furnish data. 
During the past thirty years the fresh-water pulmonates have 
been gradually undergoing the splitting process so notably carried out 
in the land snails by Dr. Pilsbry. In the middle of the past century 
the family Lymnezidz contained the genera Lymnea, Amphipeplea, 
Physa, Aplexa, Chilina, Pompholyx, Carinifex, Planorbis, Ancylus and 
Gundlachia as well as several other genera. Ancylus, Physa and Chil- 
ina with several allied genera have been raised to family rank, each 
characterized by peculiarities of shell, foot, radula, jaw, genitalia and 
other organs. Of the old Lymnzide there remain Lymnea and Plan- 
orbis, with several associated genera, each falling into a subfamily. A 
study of these two groups seems to point to their inevitable further 
separation into two families, Lymnzidz and Planorbide. The criteria 
upon which this opinion is based may be tabulated as follows: 
LYMNAIDE-. PLANORBID-. 
SHELL: long-spired. SHELL: discoidal. 
TENTACLES: flat, triangular. TENTACLES: long, filiform. 
GENITALIA: male organ with dis- GENITALIA: male organ with a large 
tinct penis and penis-sac, the former cylindrical appendage attached to the 
with a distinct, heavy retractor mus- apex of the penis. The vas deferens 
cle; penis-sac with a strong retractor enters the distal end of the appendage, 
muscle, inserted distally near the reappears at the junction of the ap- 
penis retractor. Numerous protractor pendage with the penis, and enters 
muscles present. Vas deferens in- the penis in a depression on the side. 
serted in head of penis. Genitalia on Penis with wide retractor and pro- 
right side. tractor muscles. Genitalia on left 
side. 
RabDULA: Central tooth unicuspid; Raputa: Central tooth bicuspid; 
lateral teeth bi- or tri- cuspid. 
Kiney: Very wide, short, some- 
what pear-shaped. 
lateral teeth tricuspid. 
KipNEY: Very long, narrow. 
The male system of Planorbis is very peculiar in the possession of 
the cylindrical appendage attached to the penis. (Plate XIII, A, 2.) 
This was at first thought to be the penis, but the insertion of the vas 
