LANCASTER COUNTY. 481 



Planorbis is a genus in which the shell is a flattened discoid 

 spiral; the most common species throughout the country being 

 Planorbis bicarinatus. A small species, P. parvus, is more 

 rarely found within our limits, and may be recognized by its 

 small size, being less than a quarter of an inch in diameter, 

 and by its compressed form. 



In the remaining genus of the family Physadas, the shell is 

 not spiral, but has an oval conical shape, like a shallow cup., 

 being a minute representation of the shell of the marine genus 

 Patella. It may be found attached to stones under water. — 

 The only species observed within the county is named Ancy- 

 lus rivularis. 



HELICID.E, 



The land snails have four tentacles, the principal or upper 

 pair, bearing undeveloped eyes upon their summits, and pos- 

 sessing a peculiar structure, by means of which they can be 

 withdrawn ; being tubular, the extremity turns inwards, when 

 the whole tentacle follows. Snails live under bark, logs and 

 stones, our species seldom moving abroad, except in wet 

 weather, or during the night. Our largest species is named, 

 from the broad white lip of the shell. Helix albolabris. The 

 next in size, and nearly like the preceding, is H. thyroidus, dis- 

 tinguished by a white projection or touth, upon the inner side 

 of the aperture. 



Helix trideiitata; shell depressed, base open, lip white, with 

 two teeth, opposite to v/hich, on the inner side, is a third large 

 curved tooth ; length three-fourths of an inch. 



Helix concava; shell polished, base very open, aperture 

 nearly circular, the lip expanded. 



Helix alternata; shell nearly an inch in size, open below, 

 lip sharp ; color yellowish brown, mottled with reddish bands 

 lines of growth coarse. 



Helix hirsuta may be recognised by the rough exterior, and 

 the narrow, radiating aperture, which is closed up in such 

 a manner by a large tooth, that one might suppose it difficult 

 for the animal to pass ; shell less than half an inch. 



I have observed the following species of Helix within our 

 borders, together with several others which may have been 

 collected in the neighboring counties : 



Helix albolabris ; H. thyroidus ; H. alternata; H. concava; 

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