LANCASTER COUNTY. 479 



MOLLUSCA. 



The moUusca constitute a class of animals which includes 

 all our land and freshwater shells ; but as the county is out of 

 the influence of salt Nvater, and not even touched by the tide ; 

 we are without any of the more beautiful species which occur 

 in the sea alone. This, however, should not lead us to neglect 

 these humble creatures, for they, as well as the most highly 

 organized, have had their station given to them in the great 

 scheme of creation. 



Those which construct a univalve shell, are the most highly 

 organized, and include the so called snails, whether of the 

 land or water. These move about slo\vly upon a disk called 

 the foot, in search of their vegetable food ; and instantly re- 

 tract themselves within their spiral shell, upon being distur- 

 bed. The bivalve species are enclosed in a pair of valves, 

 lined by the mantle of the animal, and closed by two strong 

 transverse muscles, thus differing from the genus Ostrea 

 (oyster) the valves of which are closed by a single muscle. 

 Upon each side of the body of the animal are two long flaps, 

 which are the gills, and the water is admitted by two siphons 

 projected a little, from the upper and hinder pait of the shell. 

 The animal moves with the open margin of the shell turned 

 down into the sand, and draws itself forward, making a furrow 

 as it advances by means of its foot, with which the oyster is 

 not provided, as it never moves from the place to which it was 

 first attached. The freshwater univalve shells have two tenta- 

 cles projecting from the head, and are divided into those which 

 breathe water, and those which breathe air ; the former have 

 the eyes situated upon an enlargement of the outside base of 

 the tentacles ; whilst in the latter, they are upon the head, near 

 the inside of these organs. At ti^e head of our Mollusca, the 

 genus llelania may be placed. It contains but a single spe- 

 cies, Melania Virginica, which occurs throughout the Susque- 

 hanna, and in many of the larger streams." The shell is an inch 

 long, with eight or ten turns ; the color green, with two spiral 

 reddish bands, in some individuals. AVith this species occurs 

 another belonging to the allied genus Anculosa, and called, 

 tr«m the dissimilarity of the various individuals, Areculosa dis- 

 similis. Length half an inch. 



