MUD-SHELLS AND BUBBLE -SHELLS. 51 



five, instead of six or seven), shorter spire, and 

 deeper suture. The foot of the animal in fontinalis 

 is rounded in front instead of lanceolate, and the 

 body is of a uniform greyish colour, instead of 

 minutely speckled as in hypnorum. Dr. Gray con- 

 siders these two generically distinct, and places the 

 latter in the genus Aplexus, pointing out that, in 

 hypnorum, the mantle has plain edges, and is not 

 expanded over the shell, which has a long spire and 

 an epidermis ; while in fontinalis the mantle is 

 lobed, expanding over the shell, which has a short 

 spire and no epidermis. But in regarding these 

 differences as specific, and not generic, we have 

 followed Turton, Forbes and Hanley, Gwyn Jeffreys, 

 and other authorities. 



Valvata piscinalis (PL VI., fig. 6) is a pretty 

 little shell, which is not uncommon in ponds and 

 still waters. The animal inhabiting it is remarkable 

 for its branchial apparatus, which is external and re- 

 sembles a plume, and for a curious appendage to the 

 mantle to facilitate respiration, which looks like a 

 third tentacle on the right side of the body. The 

 mouth of the shell is closed with an epiphragm or 

 valve ; hence the generic name, while its partiality 



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