346 THE CHICAGO ACADEMY OF SCIENCES. 
when on land preferring moist localities. Though essentially 
terrestrial, the gills are of the usual pectinated form. 
Remarks: Pomatopsis is immediately distinguished from 
the allied group Ammnicola 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 (//tstrionella 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 VALVATID&. 
“Lingual membrane with teeth in seven series (3—1—3); 
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 
ey 
Fic. .129. 
Animal and operculum of VALVATA TRICARINATA Say. (Binney, L. 
and Fr. Wat. Sh., pt. III, 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 Hydrobiine and Allied Forms,” Smithsonian Miscellaneous Collec- 
tions, No. 201, pp. 29-36. 
Land and Fresh Water Shells, N. A., Part III, p. 8. 
