THE NATURAL HISTORY SURVEY. 299 



common in some localities that several thousand may be gath- 

 ered in a few hours; the writer has observed them by hundreds 

 in tangled masses of Spirogyra. The animal is quite active 

 when temperature and external conditions are favorable. It 

 progresses by a series of "steps" or jerks and pulls the shell 

 after it, the latter being carried almost flat. Parvus is infested 

 with two species of flukes which occur in great numbers in the 

 muscular tegument of the lobes of the liver and the folds of 

 the intestines. They have been named Monostomalncanica and 

 Distoma ascoidea by Dr. Leidy (Proc. Phil. Acad., p. 200-201, 

 1877.) It has been found in the ditches at East Chicago on 

 submerged parts of Utricularia vulgaris Linne. 



Several erratic forms (monstrosities) of this species have 

 been found. Dr. H.. N. Lyon found three specimens at the 

 Chicago Avenue Water Works among some ten thousand nor- 

 mal specimens examined. The monstrosities are scalar for 

 the most part, although one specimen was normal to the last 

 half of the last whorl, when it suddenly became deflected. 

 These erratic forms seem to be very rare. 



120. Planorbis deflectus Say, pi. xxvi, fig. 6. 



Planorbis deflectus SAY, Long's Expedition, Vol. II, p. 261, pi. xv, fig. 



8, 1824. 

 Planorbis obliquus DE KAY, N. Y. Moll., p. 62, pi. iv, fig. 57, a, b, 1843. 



Shell: Small, dextral, depressed, with an obtuse keel at 

 the periphery; color light to dark horn, rarely jet black; surface 

 shining, lines of growth numerous, fine, oblique; apex not dis- 

 tinct, sunk below the level of the whorls; whorls four to four 

 and one-half,rapidly enlarging; peripheryobtuselykeeled; spire 

 flat, all of the whorls, excepting the apical, in the same plane; 

 sutures impressed; base concave; umbilicus wide, shallow, ex- 

 hibiting all the volutions; aperture suboval, deflected, much 

 wider than high (or long); peristome acute, thin, the superior 

 portion produced as in parvus, thickened on the inside; termin- 

 ations connected by a heavy white callus; interior of aperture 

 yellowish-white or brownish. 

 Lgtb., 2.00; width, 6.00; aper. Igth., 1.50; width, 2.00 mill. (12120.) 



2.50; " 5.75; " " 1.50; " 2.00 " (12120.) distorted. 

 2.00; " 6.00; " " 1.50; " 2.00 " (12362.) 



Animal: Similar to that of parvus; color blackish above, 

 lighter on base of foot. 



Jaw: As usual. 



Radula: Similar to that of parvus. In a number of exami- 



