Mollusca of Middlebury, Vt. and Vicinity. 269 



with his description, that I have not much hesitation in referring 

 them to it. 



P. elongata, Say. This species is rather common here. It is 

 rarely seen in Mass., but has been found in New Bedford by my 

 friend C. F. Shiverick, 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. The ratio is, of course, subject to some va- 

 riation, even in mature specimens, which alone should be com- 

 pared. 



Length. Breadth. Ratio. Length of spire, of aperture. Ratio. 



P.andllaria, .65 in. : .48 in. = 1.35. .1 in. : .55 in. =.18. 



P. heierostropha, .75 in. : .45 in. = 1.67. .25 in. : .5 in.=.5. 



P.gyrina? .55 in. : .25 in. =2.2. .23 in. : .33 in. =.7. 



P.elongata, .58 in. : .25in.=2.32. .28 in. : .30 in.=.93. 



Planorbis. 



p. lentus, Say, and P. corpuletitus, 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 Cham plain. 



P. campamilatus, Say. I have found this species only in the 

 Lemonfare river, where it was abundant, 



P. hicarinatus, Say. Common. 



P. armigerus, Say. Common in swamps. In the dry season 

 it takes refuge among the moist and decaying leaves. 



P. exacuoiis, \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 Champlain, but is not plenty. My friend, J. 

 W. Mighels, M. D., of Portland, has found it rather plentifully in 

 the interior of Maine. In the eastern part of Massachusetts it- 

 has been found in several places. 



P. parvus, Say. This species is common. One specimen in 

 my cabinet is \ in. in diameter. 



Vol. XL, No. 2.— Jan .-March, 1841. 35 



