THE NATURAL HISTORY SURVEY. 297 
the periphery; peristome a little thickened on the inside; inte- 
rior of aperture pearly. 
Length, 1.50; width, 5.00; aperture length, 1.40; width, 2.25 mill. (10278.) 
" 200s #1,00; fs Ce p(s iSpceabaeias F(a (12359.) 
Animal; Similar to P. parvus ; color brownish; the foot is 
very short and rounded. 
Faw: As usual. 
Radula: In all respects like that of parvus. 
Genitalia: Not examined. 
Distribution: Northern, Middle and Western States and 
Canada, south to New Mexico. 
Geological distribution: Pleistocene. 
Habitat: Found quite abundantly in rivers and ponds in 
water from four to five feet in depth, either on a muddy bot- 
tom or crawling among alge. 
Remarks: This is a distinct species easily known by its 
narrow umbilicus and very acute periphery. It is subject to 
considerable variation, and a number of names have been given 
to the different forms. Some of these forms are pure mon- 
strosities and others simple local variations. In the present 
area it is found in the southern and western region. 
SusGENus GYRAULUS Agassiz, 1837. 
“Shell: Rounded above, flat beneath, whorls few, rapidly 
increasing.” (Dall.) 
119. Planorbis parvus Say, pl. xxvi, fig. 7. ° 
Planorbis parvus Say, Nich. Encycl., pl. i, fig. 5, 1817. 
Planorbis vermicularis GOULD, Proc. Bos. Soc. Nat. Hist., Vol. II, p., 
212, 1847. (Variety.) 
Planorbis billingsit LEA, Proc. Phil. Acad., Vol. III, p. 109, 1864. 
Planorbis circumstriatus TRYON, con. HALDEMAN’ Ss Mon., p. 212, pl. 
vii, figs. 14-16, 1870. (Variety.) 
Planorbis concavis ANTHONY, Cat. of Shells of Cin. (No descr.), 187-. 
Shell: Small, dextral, depressed, with a well-rounded per- 
iphery; color bright horn to jet black; surface shining in the 
light colored specimens but dull in the black examples; lines 
of growth oblique, numerous, fine, slightly elevated, frequently 
crossed on the base by several raised spiral lines; apex not dis- 
tinct, generally, but on a level with the rest of the spire; whorls 
four, rapidly enlarging; periphery rounded; spire flat, all the 
whorls in the same plane; sutures very deeply impressed; base 
concave, sometimes with raised revolving lines; umbilicus very 
wide, shallow, exhibiting all the volutions; aperture broadly- 
