Mollusca of Middlebury; Vt. and Vicinity. 269 
with his description, that I have not idl hesitation in referring 
them to 
P. elongata, Say. ‘This species is rated common here. It is 
rarely seen in Mass., but has been found in New Bedford by my 
friend C. F. Shivasidk; Esq. 
The above four species of Physa differ chiefly in the propor- 
tions of the spire and aperture, and of the length and breadth, the 
gradation in these two particulars being parallel, as appears in the 
following table. he ratio is, of course, subject to some va- 
riation, even in mature specimens, which alone should be com- 
ength. Breadth. Ratio. Length of = of eee Ratio. 
65 1 
P. ancillaria, 65 in. : .48 in. =1.35. .1 in. == 13. 
P. heterostropha, .75in. : 45 in. =1.67. .25 in. : & in. =.5. 
P. gyrina? 55 in. :.25 in. =2.2. .23in. : .33 in.=.7. 
P.elongata, —.58 in. : .25 in. =2.32. .28in. : .30 in.=.93. 
PLanorsis. 
P. lentus, Say, and P. corpulentus, Say. These are undoubt- 
edly varieties of the same species, the former being merely a 
stunted growth of the latter. Very large and beautiful speci- 
mens were found plentifully below the falls of Otter Creek, in 
this village, during the spring of 1839, but last year not one 
could be found. Some were 1.15 in. in their greatest breadth, 
and .55 in. in the height of the aperture. This species is com- 
mon in Lake Champlain. 
“P. campanulatus, Say. J have found this species only in the 
Lemonfare river, where it was abundant. 
P., bicarinatus, Say. Common. 
P. armigerus, Say. Common in swamps. In the dry season 
it takes refuge among the moist and decaying leaves. 
P. exacuous, [exacutus?| Say. 'This species is the most de- 
pressed and fragile of all our Planorbes. A specimen .24 in. in 
diameter is only .05 in. in height, and weighs only .05 of a grain. 
It is found clinging to wood, in still water, on the margins of 
Lakes George and Chainaplnirs, but is not plenty. My friend, J. 
Ls Mighels, M. D., of Portland, has found it rather plentifally’ in 
the interior of Maine. In the eastern part of ‘Massachusetts it 
has been found in several places. 
P. parvus, se This species is common. One specimen in 
my cabinet is } in. in diameter. 
Vol. Xt, No. 2.—Jan.-March, 1841. 35 
