1899.] NATURAL SCIENCES OF PHILADELPHIA. 189 



Mesodon sections of Polygyra. It is helicoid, depressed and um- 

 bilicated, dull colored, not banded so far as known, with lunate 

 aperture and refleied peristome; parietal tooth, when present, 

 simple, not V-shaped or biramose. 



External features of the soft parts as in normal Polygyra or 

 Epiphragmophora species. In A. viiorhyssa the whole foot is gray- 

 ish white ; the tentacles and the dorsal surface of the animal ante- 

 riorly are dark plumbeous. 



Retractor muscles of buccal mass and tentacles of the ordinary 

 type, the right ocular and tentacular retractor passing between the 

 c? and $ branches of the genital system. 



cT system: Penis composed of a narrow upper portion and an 

 abruptly much more capacious lower portion; lumen with three 

 strong longitudinal ribs along the outer wall and numerous much 

 smaller folds elsewhere, no papilla or distinct pilaster. Penis 

 retractor muscle very short, inserted upon the epiphallus near its 

 lower end, a portion of it passing to the apex of the penis. Epiph- 

 alius distinctly diflferentiated from both penis and vas deferens, very 



