FRESH-WATER SHELLS. I 3 



genera: Sphcerium corjieiim, Sphcerium laatstre, Pisi- 

 diitni fontinale^ PisidiiLin pusilluin var. obtusalis, Pla- 

 norbis iiitidus, Planorbis nmitileus, Liumcea peregra, 

 Limncea stagnalis^ and Ancylus laciest ris. 



Ponds at Finchley, Middlesex. — Near the middle of a 

 meadow at Finchley (by the side of the foot-path to 

 Hendon, and in the third field from the road at East- 

 end, Finchley)^ are two ponds, without inlet or outlet, 

 which lie near together upon a ridge slightly higher than 

 the surrounding ground, perfectly isolated from other 

 water, and which, it seems certain, can never have been 

 affected by floods ; they are evidently artificial, but 

 probably very old, and neither is of large size, the larger 

 measuring about 29 x 10 yards, and the smaller about 

 14 X 7 yards. In the case of ponds near large towns 

 or cities having many shell collectors one can never be 

 absolutely certain that molluscs have not been intro- 

 duced intentionally or unintentionally by human agency, 

 but it is unlikely that this often happens, and I know 

 of no reasonable ground for such a suspicion in the 

 present case. Ivy- leaved duckweed {Lemna trisidca)^ 

 water-thyme {Elodea canadensis), frog-bit {Hydrocharis 

 morsus-rancB), and a Potamogeton are the principal 

 water-weeds, and there are none of kinds likely to have 

 been planted by man. Molluscs occur in abundance, 

 the bivalve Sphcerium corneuin being plentiful in the 

 mud, and very fine. In the larger pond four species, 

 belonging to four genera, are found, all in good numbers : 



1 This field, I find, is already being cut up for building 

 purposes. 



