162 



The above table points out some interesting facts : 

 whilst Anodonta cygnea var, Ponderosa weighs 1590 

 grains, Caiicbiuin miuimum only weighs the 133rd part 

 of a grain; thus it requires 211,472 specimens of the 

 latter to weigh as much as one specimen of the former. 



A second fact shown, is that certain species attain a 

 much greater weightinonelocality than in another. Thus 

 Limnoeus stagnalis is double the weight in a sheet of 

 water in Hartwell Park [Bucks.], to what it is near 

 Nottingham ; in the former place being 22 grains, 

 whilst in the latter it is only 11. A variety of Helix 

 caperata, found near Norwich, weighs nearly double 

 what that species does as found here. 



Balea fragilis appears to flourish best in our colder 

 northern counties, for at Penzance 17 specimens are 

 required to weigh one grain; near Nottingham, 13 j 

 and at Reeth, near Richmond, Yorkshire, only 8. 



The difference in the weight of two species found in 

 the same locality is so great, that it is possible that, 

 instead of proving only varieties, as now considered, 

 ihey may eventually be found to be two distinct species. 

 The first is, Pbysa fontinalis, the large var. weighing a 

 quarter of a grain, and the small var. only a tenth of 

 a grain. The second is Helix nemoralis, weighing 22 

 grains, whilst the var. Hortensis only weighs 8 grains- 

 A pure yellow var. of H. nemoralis, almost equal in 

 size to the common form of H. nemoralis, weighs 14 

 grains. The weight of some species is considerably 

 greater than others of much larger size ; thus Neritina 

 fluviatilis weighs 2 grains, whilst Planorbis carinatus 

 oaly weighs one grain. It takes 25 specimens of 

 Vitrina pellucida to weigh a grain, whilst only 16 are 

 required of Papa umbilicata to weigh the same. 



The above table, although made with great care, and 

 the scales employed of excellent adjustment; yet, no 

 doubt some slight alterations may be required, 



