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 Sfervogyra. 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 Monostoma lucanica and 
Distoma ascoidea by Dr. Leidy (Proc. Phil. Acad., p. 200-201, 
1877.) It has been found invthe ditches at East Chicago on 
submerged parts of Utricularia vulgaris Linné. 
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, pl. xxvi, fig. 6. 
Planorbis deflectus SAY, Long’s Expedition, Vol. II, p. 261, pl. xv, fig. 
8, 1824. 
Planorbis obliguus DE Kay, N.Y. Moll., p. 62, pl. 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; periphery obtusely keeled; 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. 
Lgth., 2.00; width, 6.00; aper. Igth., 1.50; width, 2.00 mill. (12120.) 
Penne So oh cee kate eee AEE TBs es DOG, (12120.) distorted. 
DRM: tana co CRM err mR Ln ed tO 2 (12362.) 
Animal: Similar to that of parvus; color blackish above, 
lighter on base of foot. 
Faw: As usual. 
Radula: Similar to that of parvus. In a number of exami- 
