190 PROCEEDINGS OF THE 



and Vahata piscinalis ; then Valvata cristata, Planorbis nitidus, and 

 Limrma glabra, and last of all Limncea auricularia. 



It appears, so far as I have investigated the ten mile area, 

 that its existing aquatic mollusca consists of 11 genera and 22 

 species, viz., — 5 genera and 8 species of Lamellibranchs, 2 genera 

 and 3 species PerJinibranchs, and 4 genera and 12 species Puhnono- 

 branchs, while the terrestrial (not including the Limacidce) consists 

 of 8 genera and 24 species, all of which are Pidmonobranchs. Thus, 

 while the aquatic molluscs exceed the terrestrial by three genera, the 

 terrestrial exceed the aquatic by two species; showing thereby in the 

 aquatic a greater generic diversity of structure than in the terres- 

 trial; and in the terrestrial, a greater specific diversity of structure 

 than in the aquatic. However, in comparison to the number of 

 genera and species, I think these differences are small, and it 

 seems to me remarkable that the two kinds should approximate 

 so closely in number, more especially when we consider that all 

 classification is necessarily more or less arbitrary; and probably 

 this is a proof of the correctness of the investigation of 

 those naturalists who arranged these mollusca into genera and 

 species, as it appears more in harmony with the laws of nature, 

 that these two great classes should nearly balance other with 

 regard to numbers, than that the one should greatly surpass the 

 other in the same respect. 



I now come to the second part, "\dz., Jflien to collect. — As both 

 land and fresh water mollusca hybernate, it would be comparatively 

 a waste of time to attempt collecting either early in spring or late 

 in autumn. I have tried it at both periods, and have no hesitation 

 in saying that I could find as many species in a week, about 

 the end of June, as could be found from the beginning of Septem- 

 ber till the end of April; besides, supposing that during March 

 and April there are some fine days, the specimens which you may 

 then find are mostly young, and the shell therefore only partly 

 formed; or should you secure adults tliey are often empty, or, if 

 alive, much weatherworn, the epidermis being either entirely 

 rubbed ofi", or so defaced as to spoil their appearance. I would 

 therefore say that the most profitable as well as pleasant time for 

 collecting is from the beginning of June till the middle of August; 

 but even during this interval, should the weather be dry and hot, you 

 will make little progress with the land shells except in the early 

 morning or late in the evening, at which periods the vegetation is 



