FRtlSlTWATETl MOLLtTSKS. 9 



shall not find in freshwater ; but yon are most of yon familiar with 

 8omo of them, such as the Ammonite, Belomnite, Squid, and Cuttlefish. 

 And lastly (cj, the Gastropoda, creatures that creep by means of a 

 muscular disc attached to their belly, such as the slug and snail. It is 

 with this class, and with those members of it called the freshwater 

 snails, that we have the most to do to-day. 



We shall, however, find a shell named Succinea, which although an 

 air-breathing and not a water-breathing mollusk, is never found except 

 in wot places. I shall hope to point it out to you presently. The 

 genus Succinea contains three species. 



Next we come to four families of shells, the inhabitants of which 

 breath both air and water, and can, consequently, live on mud and on 

 the banks of rivers a short time, although water is their more congenial 

 element. 



1st. Pla-vyrhis, shaped somewhat like an Ammonite, of which there 

 are eleven British species, and of which we may hope to find some 

 specimens, at any rate Planarbis complanat^lS, which is flat one side and 

 has a sharp keel ; and, perhaps, Planorhis crista, a small, dehcato, ribbed 

 shell, with which I made my first practical acquaintance in this canal 

 about five weeks ago. 



2nd. Physa, of which there are two species : always coiled to the 

 left, as you will see if you find a specimen by holding it with the aper- 

 ture facing you and the apex or spire upwards : the aperture then will 

 appear on the left hand side. The common snails are coiled to the 

 right, and a left-handed garden snail would be a rich prize as there are 

 not a dozen in the British Museum. 



Physa may be easily recognised by its body being much too large for 

 its shell. 



Its shell, too, is very bright, its occupant continually polishing and 

 cleaning it by portions of its body folded up over its shell like little 

 fingers ; we may take it home as a model for housewives, if we can 

 succeed in fijiding a specimen. 



Physa has the property of letting itself down from the top of the 

 water, or from the leaf of a water plant, by a thread of mucus. 



This has been doubted by several able authorites, among them the 

 late Lovell Reeve ; but I have had the good fortune to witness it my- 

 self, and experimented on it in the presence of some friends with such 

 conclusive results that the above-mentioned eminent conchologist ex- 

 pressed himself perfectly satisfied when I gave him an account of my 

 investigations. 



3rd. Ltjrrmosa, the commoner family of water snails, of which we 

 have eight in Great Britain, of which one is confined to Ireland and 

 two others are very i-arely met with. 



We shall find, I hope, to-day, at any rate two species, L. limosa, also 

 known as L. peregra ; and L. palustris, a shell easily recogised by the 

 malleated appearance of its surface as if it had been hammered all 

 over. Nor do I see any reason, except in the fact that 1 have not yet 



