LYMNiELLVE 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 Lymnseas 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 Lymnseidse contained the genera Lymncea, Amphipeplea, 

 Physa, Aplcxa, 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 Lymnseidse there remain Lymncea 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, Lymnseidse and Planorbidse. The criteria 

 upon which this opinion is based may be tabulated as follows : 



lymn^im:. 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. 



Radula : Central tooth unicuspid ; Radula : . Central tooth bicuspid ; 



lateral teeth bi- or tri- cuspid. lateral teeth tricuspid. 



Kidney : Very wide, short, some- Kidney : Very long, narrow, 

 what pear-shaped. 



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 



