THE FRESH-WATER AQUARIUM. 77 



Xew York species. The color is a pale greenish or straw 

 yellow. It was when keeping this species that I first be- 

 came acquainted with the mode of growth of the young 

 from the egg to maturity. The process I will describe 

 when we come to speak of the Physa hdcrostropha. 



jAjmnca fragilis (PI. V., Fig. 4) is another species of 

 this genus that I have introduced with advantage into 

 my fresh-water tank, and is perhaps a more graceful shell 

 in form than the last mentioned. It is darker in color 

 than the L. columella, but is not so large a shell, being 

 only from six to nine-tenths of an inch in length. It is 

 the same as the L. fragilis of Europe, and is common in 

 this State. The reader, in his rambles, no doubt will come 

 upon many other species .of this genus, all of which may 

 be used for the fresh-water tank. The commonest, however, 

 in the southern part of the State of New York, arc the 

 above and L. desidiosa. 



The genus Physa, which are left-handed shells — that is 

 to say, when the opening is held towards us, it is on the 

 left hand of the apex, whilst in the genus Lymnea it is 

 on the right hand — are represented in New York State 

 by eleven species. 



Physa hekrostrophu (PI. V., Fig. 5) is a very common 

 species, found in almost every pond and stream. It is of 

 a yellowish or greenish-yellow color, and becoming more 

 dusky with age. It has the inside of its lip of a dull 

 reddish color. This animal, as well as the Lymnea, is often 

 seen floating, with its shell downwards, upon the surface 

 of the water, literally, as a writer has observed, " walking 

 on the surface of the air," and moving along by means, 



