FRESH-WATER SHELLS. I 5 



and has never been seen to reach a small stream which 

 runs along at the bottom of the field, perhaps 150 yards 

 away. The shell-fauna, it will be seen, is a wonderfully 

 rich one, comprising, as it does, eleven species belonging 

 to six genera, including two bivalves and no less 

 than five kinds of coil-shells or Planorbes. But the 

 pond is additionally interesting from the fact that 

 a number of species seem to have been introduced 

 quite recently. During 1860-3, when Mr. Nelson was 

 in the habit of collecting there, it yielded only four 

 species, Sphcerium lacustre, Pisidimn piisilbim, Planor- 

 bis natitileus, and Lhuncsa peregra^ so that since that 

 period its fauna seems to have received a surprising 

 number of additions. About 1873, Mr. Nelson re- 

 sumed his visits to the locality and found Planorbis 

 Cornells^ but no further kinds were obtained till 1883, 

 when during frequent visits, he found six others, Physa 

 fontinalis and Planorbis vortex in the spring, Planorbis 

 carinatus^ Limncea stagnalis, and Ancylus laaistris a 

 little later, and finally, in June, this small, but prolific 

 pond yielded its fifth Plaiwrbis^ PL conioi^tus. Mr. 

 Nelson, who has published these facts,^ is a collector of 

 very great experience and a naturalist of ability, so that 

 I dare not suggest that all the shells ultimately found 

 probably existed in the pond, overlooked, during 1860-3, 

 and Mr. Taylor and Mr. J. Beevers, who collected with 

 him, agree that the four species named were the only 

 forms in the pond at that time. The case is at least 



1 W. Nehon, " Journ. of Concli.,'^ iv. (1883), 117. 



