[whittaker] bottom DEPOSITS OF McKAY LAKE 155 



molluscs inhabit this area. As might be expected plants are few as 

 the influx of new material each year tends to stifle and kill off the old 

 ones. Here Valvata tricarinata, Amnicola porata, and Pisidium abdi- 

 tum are the only forms regularly present. Ancylus was noted once on 

 a dead Physa shell and Planorbis campanulatus and P. bicarinatus were 

 rarely found but apparently were casual forms which had wandered 

 in from the preceding habitat. An interesting feature about the 

 former species was the manner in which they had by living on the 

 dark clay and rock fragments, incorporated the colours of these 

 materials into their shells. The epidermis of Valvata and Amnicola 

 in the preceding habitat is a delicate or pronounced shade of green, 

 here it is black. The colour does not seem to be merely in an external 

 coating, but extends deep into the shell. 



In the blackish brown mud of zone 4 practically all molluscan 

 life had ceased. There were dead shells in abundance, and most of the 

 large shelled Lampsilis, and Anodonta. A single individual of Lamp- 

 silis radiatus was found dead but in a natural position as though it 

 had died there. It was brought up in a haul of the orange-peel bucket. 

 Plant life except of microscopic forms was absent. 



In zone 5 — the red ooze — ^very rarely a shell of Lampsilis 

 radiatus was brought up. In each case the shell was badly decom- 

 posed. The lime was dissolved, leaving behind the thin flexible chi- 

 tinous epidermis. , 



In zone 6, a restricted area at the southern end of the lake, there 

 is a sparse plant growth upon a sandy bottom. Here Campeloma 

 decisum, Planorbis antrosus, and Lymnaea stagnalis appressa were 

 found, in considerable numbers. This is the only place in the shore 

 where there is a sand beach and since it is subjected to the small amount 

 of wave action in the lake, many hundreds of dead shells are piled up 

 at the water's edge. In the east bay where the bottom consists of 

 a substratum of marl, few plants are present and the molluscan fauna 

 is rare, consisting almost entirely of Amnicola porata and Valvata tri- 

 carinata. 



In passing it may be said that for the most part, owing to the 

 manner in which the recent deposits have encroached upon the lake, 

 there is practically no bottom area exposed between water level and 

 1' in depth, as the clumps of grass sedge and peat go down vertically 

 to a depth of a foot or more. In many lakes important differentia- 

 tions can be made in the fauna for this zone of - 1' but this zone 

 cannot be distinguished here. 



