MOLLUSCA. 125 



widening it by meandering greatly, so that quite extensive creek flats 

 have l)een and are being formed. 



Rush Lake. — Rush Lake occupies a semi-circular depression in the 

 clay plain immediately behind the sand dune area. On the northern and 

 northwestern .sides of the lake is the innermost ridge of sand dunes, 

 while on the southern and southwestern sides is a portion of the clay 

 plain, similar to but not .so well drained as around Caseville. where the 

 land is .somewhat higher and slopes toward the Pigeon River. 



Geographic Relations. 



Aquatic Forms. — The portion of Huron County under consideration 

 was fir.st uncovered after the Wisconsin stage of glaciation at about the 

 time when Lake Warren combined into one lake what had been Lake 

 Saginaw, a lake of the Saginaw Bay basin, and Lake Whittlesey, a lake 

 of the Huron-Erie ba.sin. At, and previous to, this time, the Great 

 Lakes had been draining into the Mississippi valley, and it was not until 

 considerably later that they began to empty into the Atlantic Ocean. 

 The Saginaw-Grand River drainage led directly past here, and this and 

 others following nearly the same route were the main ch-ainage 

 channels, altho Lake Maumee, one of the progenitors of Lake Whittle- 

 sey, early in its life hi.story drained thru the Fort Wayne outlet into the 

 Ohio.* 



Thus the acjuatic habitats in Huron County are at. present in connec- 

 tion with the northern and southeastern portions of the state, and have 

 been connected with the western portion. It is then not surprising to 

 find that the aquatic shells of Huron County are mostly similar to those 

 of all parts of the lower peninsula. The strongest affinities appear to 

 be with the northern and southeastern portions; for example, Lamp- 

 silis multiradiata, which is found only in the southeastern portion of the 

 state, inhabits the Pigeon River, while Sphaerium flovum, more typical 

 of the northern portion, is common in Rush Lake. On the other hand, 

 no shells distinctive of the western portion appear to occur in the region 

 stucUed.t 



Terrestrial Forms. — The land molluscan fauna of this region has rather 

 distinct northern affinities. Altho those collected with the excep- 

 tion of six forms, are those of general distribution at least in the 

 lower peninsula, the smaller shells which are mo.st characteristic of the 

 northern regions were more prevalent here. Of the six exceptions, 

 four (Vitrea hinneyana, Zonitoides milium. Pyramidula cronkhitei 

 catskillensis. and Cochlicopa luhrica morseana) ar,e northern forms, 



*A. C. Lane, Geol. Rept. Huron County. Geol. Surv. Mich., Vol. VII, Pt. II, Chap. IV. 

 tWalker. Bryant. The Di-stribution of the Unionidae in Michigan. Detroit. Mich., 1S9S. 



