282 MICHIGAN SURVEY, 1908. 



Pisidium affine Sterki. 



Pisidium sargenti Sterki. 



Pisidium roperi Sterki. 



Pisidium suhrotundmn Sterki 



Pisidium splendidulum Sterki. 



Pisidium pauperculum Sterki. 



Pisidium medianum Sterki. 



Pisidium punctatum, simplew Sterki. 



Of these Lynncea pilshryana is, so far as yet known, peculiar to Isle 

 Royale. The remainder have, as a rule, a general distribution all over the 

 state. StroMlops virgo and Zonitoides exigua, however, are characteris- 

 tic boreal species and are apparently rarely found south of the Saginaw- 

 Grand Valley. While, of course, it is possible that Isle Royale marks 

 the northern limit of the range of these species, in view of the fact 

 that nearly all of them are known to range through the Upper Peninsula 

 and that practically nothing is known of the fauna of the north shore 

 of Lake Superior, the probability is that most of them range further 

 north and should be included in the boreal fauna of North America. 

 While the Isle Royale fauna is thus to be considered purely boreal 

 in its character, it must be understood that it is not in any materia! 

 respect different from that of the Upper Peninsula and of the northern 

 part of the Lower. The occurrence of such species as Vallonia costata, 

 Vertigo tridentata and Physa aplectoides must be considered rather as 

 evidence that these forms have been overlooked in the region immediate- 

 ly south rather than that they are restricted to Isle Royale. And the 

 existence of Lymnwa piWbryana and the peculiar forms of Lymnwa 

 stagnalis, PlanorMs licarinatus and P. campanulatus, if ultimately found 

 to be confined to the island, should rather be ascribed to long isolation 

 under peculiar local conditions than as indicating any essential difference 

 from the fauna of the surrounding region. 



Considering the, fauna of the Upper Peninsula and Isle Royale as a 

 whole, it will be- found to include two elements. The first and larger 

 one consists of species having a general range through the northern 

 United States and southern portion of Canada. Just how far to the 

 north most of them range is as yet undetermined. These species, although 

 found in the boreal region, cannot be properly said to be distinctly 

 boreal species. Accompanying these species of general distribution, is 

 a smaller element of purely boreal species, which are characteristic of 

 the northern region and whose range to the south is as a rule quite 

 restricted. This purely boreal element is represented on Isle Royale 

 by the following species : 



1. Acanthinula Jiarpa (Say). 



2. Vitrina limpida Gould. 



3. Vitrea hinneyana '(Mse.). 



4. Zonitoides exigua (Stimp.). 



5. Pyramidula asteriscus (Mse.). 



6. Lymnwa megasoma Say. 



7. Lymnwa emarginata Say. 



8. Lymnwa pilshryana Walker. 



So far as the land species of Isle Royale are concerned, there are 

 but few of special interest, most of them being of general distribution 



