346 THE CHICAGO ACADEMY OF SCIENCES. 



when on land preferring moist localities. Though essentially 

 terrestrial, the gills are of the usual pectinated form. 



Remarks: Pomatiopsis is immediately distinguished from 

 the allied group Amnicola by the peculiar "stepping" mode of 

 progression. This is accomplished as follows: The anterior 

 part of the foot is placed firmly on the ground, then the poste- 

 rior part is pulled forward; next the rostrum is firmly attached 

 to the ground and the posterior part is also attached, allowing 

 the anterior part of the foot to be placed forward. In this 

 manner the mollusk "steps" along at a lively gait. The author 

 has seen this species traveling in the water with the rostrum 

 bent downward, moving about like a hound on the scent.* 



A species of worm (Histrionella pomatiopsidis Stimpson) 

 infests the branchial cavity of Pomatiopsis. 



The species is confined to the southern and western regions, 

 and seems to be quite rare. It is sometimes found associated 

 with P. cincinnatiensis . See remarks under that species. 



FAMILY VALVATIM;. 



"Lingual membrane with teeth in seven series (313); 

 the central tooth broad, with hooked and denticulated apex, 

 the laterals lanceolate-hooked and denticulated. Rostrum 

 produced; tentacles cylindrical, eyes sessile at their exterior 

 bases. Mantle simple in front; gill plumose, exposed, the 



FIG. 129. 



Animal and operculum of VALVATA TRICARINATA Say. (Binney, L. 

 and Fr. Wat. Sh., pt. Ill, Figs. 10, 11. 



lamina primate, spirally twisted, protected by a long, slender 

 respiratory lobe. Foot bilobed in front. Operculum horny, 

 orbicular, spiral, many whorled; whorls with a thin, elevated 

 edge (Fig. 129). Shell spiral, turbinate or discoidal, covered 

 with an epidermis; aperture with the peritreme entire." (Bin- 

 ney.)t 



*For a full account of the "stepping" or "looping" of Pomatiopsis, see Stimpson's 

 "Researches upon the Hydrobiinse and Allied Forms," Smithsbnian Miscellaneous Collec- 

 tions, No. 201, pp. 29-36. 



tLand and Fresh Water Shells, N. A., Part III, p. 8. 



