126 MOLLUSCA. 



while two (Strobilops virgo and Bifidaria cortkaria) are typical of the 

 northern and middle counties. 



This northern element in the fauna may be due to the fact that the 

 pecuHar shape and location of this county make it a sort of a pocket 

 which prevented the shells entering it after the retreat of the glaciers 

 from migrating farther northward, but this cannot be jDOsitively ascer- 

 tained until more is known of the interior of the county. Only the re- 

 gion among and near the sand dunes was studied, and the.se conditions 

 and those of the adjacent swamps are often favorable to northern faunas 

 and thus may have formed a region of preservation of these forms.* 



HABITAT .STUDY. 



Aquatic Habitats. 



Habitats of Saginaw Bay. — 1. Deep littoral, 16-24 feet. This is just 

 such a habitat as was found by Hankinson at corresponding depths in 

 Walnut Lake,t ))ut in -this case the habitat does not extend so far in 

 towards shore, as the instability of the sand in shallower water effect- 

 ually prevents all higher plant and most animal life. The only data in 

 regard to this habitat was obtained from the evidences of life which 

 were brought up on the line of fish nets off the camp on Sand Point, 

 when these were taken out for the summer. It was found at this time 

 that the sandy bottom at these depths has a scanty vegetation, con- 

 sisting, in the main, of Potamogeton. This belt of vegetation begins in 

 about sixteen feet of water, is most dense in about nineteen feet, and 

 is diminishing at the end of the net in about twenty-four feet of water, 

 one and three-quarters miles off shore. Altho this is the only locality 

 actually studied, it was deduced from dead shells along the shore that 

 this kind of a habitat stretches all the way along the north shore of Sand 

 Point. 



In this habitat were obtained a few specimens of Goniobasis livescens, 

 one of Campeloma decisum. one of C. integrum, seven of C. rufum. a 

 single light-colored specimen of Planorbis bicarinatus, two specimens 

 of a variety of Physa ancillaria (one of which was very much distorted),' 

 and a specimen of the lake form of Lampsilis luteola. Two other species, 

 Pkniorbis truncatus and Lymnaea catascopium. which had been washed 

 up on the beach were referred to this hal)itat. as there was apparently 

 no other habitat in the vicinity from which they could have come. 



2. Shallow littoral, rocky, unprotected. In places, especially at the 

 ends of points facing the north and on the north sides of islands, the 

 sand is washed away, or perhaps has never been deposited, so that the 



*Adams. C. (". Post Glacial Origin and Migration of Life. Jour. Geog., Vol. I, 1902, p. 310. 

 t State Geol. Siirv. of Mich. Ann. Rept. 1907. 



