MOLLUSCA. 133 



lakes were apparently formed in the same manner as one is even now 

 being formed at Turtle Ba}' (habitat 8). 



These habitats are much richer in plant and animal life than the sand 

 dune lakes along the north shore of the Point. Yellow and white water- 

 lilies abound, and Potamogetons are also very numerous. The common 

 rush (Scirpus americanus) is largely replaced by the bulrush (Scirpus 

 validus) ai)d cat-tails {Typha latifolia), the first named appearing to 

 prefer sandy regions. The alders have deposited a layer of fallen leaves 

 in the shallow places along the shore; these are the places that were 

 most studied. 



The most abundant shell in this habitat was Planorhis campanulatus; 

 both the ty])ical shell and a variation of it were obtained. Planorhis 

 exacuous, P. trivolvis, and a form of Segmentina crassilabris were also 

 found in considerable abundance. 



All along this side of the Point are many large swamps and peaty 

 bogs. These are apparently the beds of former lakes, which have been 

 filled in by the encroachment of vegetation and are probably the final 

 stage in the life history of lakes similar to Mud and Orr Lakes. 



Habitats of Rush Lake. — Rush Lake is about three miles long by one 

 wide and lies just behind the sand dunes farthest from Saginaw Bay. 

 Its formation was probably due to a combination of the forces acting 

 in the formation of the lakes on Sand Point; also, the lowering of Lake 

 Huron, mentioned in the preHminary discussion, probably played a 

 greater role in the production of this larger and older lake. It can not 

 loe included among the sand dune lakes, as it is not entirely surrounded 

 by the sand dunes; its greater size and age also tend to place it in a 

 class by itself, as does its fauna. (Pis. IX, X a.) 



The greater portion of the lake has a soft, mucky bottom, but in 

 some places there is a heavy deposit of marl which makes the bottom 

 more firm. In other places, off, and to the west of, points running out 

 from the northwest shore, the sand or gravel outcrops or has only a very 

 thin covering of peat and marl. There are three such places — off Ray- 

 mond's, Dune and Fourth Points. 



In mucky-bottomed places, there are three more or less well-defined 

 zones. Counting out from the shore these are: the sedge-marsh zone, 

 the water-lily zone, and the pond-weed zone. Thruout the other por- 

 tions of the lake, the sedge-marsh is broken thru, and then the three 

 zones are: the zone of Scirpus americanus, the zone of Scirpus validus 

 (the bulrush), and the mucky zone with a vegetation consisting of horn- 

 worts, Potamogetons, etc. Between the two la.st there is often a zone 

 with no vegetation. 



This lake is a good example of the encroachment of vegetation in a 

 filling lake. Besides the sedge marsh, other water plants have built 



