
MOLLUSCS, VERMES AND HYDROZOA 

with lines of growth, with the body whorl slightly depressed. The suture 
is distinct, the apex compressed and the aperture dilated and deflected to 
the left; the other side showing the whorls nearly as well defined. The 
body is a dusky brown or russet, and the filiform tentacles are long and 
marked with dark brown lines. It inhabits streams of colder water in 
New England, New York, Northern Pennsylvania, Ohio and Illinois, and 
has been occasionally met with in the vicinity of Philadelphia. 
P. trivolvis, Fig. 170, is a very generally distributed species having a 
laterally flattened 34 to 7% inch in diameter yellowish-green or brown shell, 
which consists of four and a half cylindrical whorls with finely marked lines 
of growth, and is slightly keeled towards the left side. The aperture is 
also deflected to the left. The spire is slightly impressed, nearly level on 

FIG. 170. Planorbis trivolvis, four views. Slightly enlarged. 
disappear in a depression about two and a half whorls from the apex. 
The body is dark brown dotted with ochre and the tentacles long and 
slender. This snail occurs very generally in the Eastern and Middle 
States and is found in the Delaware and Schuylkill rivers. Its eggs are 
laid in a yellowish vermiform mass and hatch in 12 to 20 days. 
P. magnificus, Fig. 171,is the largest recently known American species, 
having the flattened sinistral shell very large and heavy, about 1 % inch in 
diameter and 1 inch high. The upper or spire half of the shell is pale- 
brown and the lower half dark-brown. The surface is glossy and marked 
by fine lines of growth. The spire is narrow, the suture depressed, and 
the summit of the nearly five complete whorls acutely angular and the 
umbilicus deeply funnel-shaped. The base of the whorls is so narrowly 
rounded as to appear almost angular. The last whorl is very large, rounded 
at the periphery, and the irregularly ovate aperture but slightly oblique. 
234. 
