ECOLOGY OF ISLE ROYALE. 63 



164), but the imagos were common all along the shore of the lake, es- 

 pecially on the gravel beaches. One (No. 192) was taken on the boat 

 about 2 km. off the south shore of the island. Stone flies were also fre- 

 quently collected along the beaches, where they came up to breed. They 

 were most numerous, however, on steep or even vertical cliffs with south- 

 ern exposure (Nos. 24, 80). Near the entrance to Conglomerate Bay 

 (Fig. 2) they were seen collected in such a place by thousands. The 

 water there was at least 4 m. deep. 



A few hair-worms, Gordms aquaticus (L.) (No. 207), were collected 

 in 2 to 3 dm. of water on the rock beach {Fig. SO) in front of the camp 

 on Siskowit Bay. 



The various mussels collected in Siskowit Lake and elsewhere, even 

 though sometimes associated with Limnaea stagnalis, belong rather to 

 the associations of the smaller inland lakes. 



III. The Inland LaTce. 



The smaller lakes are mainly surrounded by tamarack swamps, with 

 the vegetation showing the characteristic zones, certain ones of which, as 

 the rushes, water-lilies and pond weeds, live in the lake itself. The bot- 

 tom is covered with peaty mud or with slime, and the wave action is 

 never severe enough to interfere with the growth of either fauna or 

 flora. In many of the smaller lakes, in fact, the water lily zone is so 

 wide and the open water so restricted that there is practically no wave 

 action at all {Fig. Jf6). Accordingly both fauna and flora are 

 richly developed both in species and individuals. The fauna may 

 be roughly classified into several groups according to their habitat in 

 order to facilitate description. The interrelations of the different spec- 

 cies are complex in the extreme, and of course could not be properly 

 worked out in such a short time as the lakes were under observation. 



a. The Fauna of the Bottom. In Sumner Lake (III, 5) {Figs. 18-22' 

 and in sheltered places in Siskowit Lake several species of shells live on 

 ihe bottom in sand or mud and at a depth of from 3 dm. to 1 or 2 m. 

 Planorhis trivolvis Say (No. 135) lives in the shallower water, prefer- 

 ably in mud. It is nowhere abundant, but was collected in both lakes. 

 One specimen only was found in Sumner Lake in a little pool with mud 

 l)ottom. Shells were commoner on the shoreward side of an island in 

 Siskowit Lake, on a bottom composed of sand and mud. They were 

 well buried under the sand and the majority of the shells were dead. 



Mussel shells, especially Anodonta marginata Say and Anodonta 

 grandis Lea, were common in all the smaller lakes and at the upper 

 end of Rock Harbor. They were most abundant in the deeper water 

 with a sand bottom, particularly where there was comparatively 

 little vegetation. In certain sheltered bays at the upper end of Sisko- 

 wit Lake they were especially numerous. Muskrats carry them to the 

 shore to eat, and leave the empty shells in heaps, which were conspicu- 

 ous sights along most of the lake shores. In Sumner Lake live shells 

 were very scarce, but the piles of dead ones on the bank testified to their 

 former abundance. 



At the upper end of Eock Harbor some small shells, PlanorMs hicari- 

 natus Say (89), PlanorMs exacutus say (89), Plwnoi'his parvus Say 

 {89, 163, 164), Yalvata tricarinata Say (89, 163), Yalvata sincera ny- 



