POND AND EIVEK SHELLS. 45 



species in all can be fairly recognized. Short of 

 pointing out in what respects these six (or rather 

 five, as British) differ inter se, we have, perhaps, 

 said enough to stimulate research on the part of 

 the collector. 



The aquatic bivalves, then, may be thus enu- 

 merated three Unios, two swan mussels, one zebra 

 mussel, four species of Sphaerium (or Cyclas), and 

 five of Pisidium, or fifteen species in all. The 

 aquatic univalves, or pond snails as they are often 

 termed, are much more numerous, though the 

 majority of them belong to two well-marked but 

 very different genera the flat coil- shells, Planorbis, 

 and the thin - shelled, long -spired mud -shells, 

 Limncea the remainder belonging to seven other 

 genera, to be named presently. The various species 

 of Planorbis, of which some six or seven are to be 

 met with around London, frequent ponds, ditches, 

 marshes, and stagnant water, and are generally 

 found floating on the surface, or adhering to duck- 

 weed and the leaves of other aquatic plants. Of 

 the largest and most remarkable species, P. corneus 

 (PI. VI., fig. 4), we have already spoken (p. 16) 

 when referring to the more noticeable generic forms 



