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MOLLUSCA, CRUSTACEA AND KEPTILIA FOE 



Mollusca must bo looked for, as we have before sug- 

 gested, in greater quantity and beauty in the ocean than 

 in fresh-water lakes aud rivers. The marine shells also 

 present us -with a greater variety in form than our 

 fresh-water friends, but many of those that wc find in 

 fresh- water streams and milldams are of great beauty and 

 grace. The pretty coil shell rianorbis, the Lymnea, Physa, 

 Paludina, Malania and fresh-water muscles, as they are 

 called, the Unios, and Anodons all look well and are ex- 

 tremely useful in a fresh-water tank, as removers of the 

 troublesome -decaying matter. 



The Mollusca, or shell fish, arc divided into two great 

 groups : the univalves, or those having the shell formed in 

 one piece ; and, the bivalves, or those having the shell con- 

 sisting of two portions "united together at some ouc point 

 by a hinge. 



Of the first class, we have several that may be intro- 

 duced with advantage into our tank ; and first among 

 them comes the genus Lymnea, of which there arc fifteen 

 species found in the State of New York, some of them 

 being very common. 



Lymnea columella (PI. V., Fig. 1). I have found this 

 species very common in several places. It is an ovate, fra- 

 gile shell, varying from one half to one inch in length, 

 though it is generally found smaller than that. The spire 

 is narrow, acute and much shortened at the aperture. 

 Aperture very large, more or less expanded, and haviug a 

 thin film of enamel at the lip. This tip is also so much 

 arched as to display a considerable portion of the interior 

 of the shell, which distinguishes it from any other of the 



