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.) 

 2.00; " 7.50; " " 1.75; " 3.00 " (12359.) 



Animal: Similar to P. parvus; color brownish; the foot is 

 very short and rounded 



Jaw: 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 algae. 



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. 



SUBGENUS QYRAULUS Agassiz, 1837. 



"Shell: Rounded above, flat beneath, whorls few, rapidly 

 increasing." (Dall.) 

 119. Planorbis parvus Say, pi. xxvi, fig. 7. 



Planorbis parvus SAY, Nich. Encycl., pi. i, fig. 5, 1817. 



Planorbis vermicularis GOULD, Proc. Bos. Soc. Nat. Hist., Vol. II, p., 



212, 1847. (Variety.) 



Planorbis billingsii LEA, Proc. Phil. Acad., Vol. Ill, p. 109, 1864. 

 Planorbis circumstriatus TRYON, con. HALDEMAN'S Mon., p. 212, pi. 



vii, figs. 14-16, 1870. (Variety.) 

 Planorbis concavus 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- 



