122 MOLLUSC A. 



in the field, others from the .specimens after the party had returned to 

 Ann Arbor. In this connection it should be said that the majority of 

 the identifications were made or verified by Mr. Bryant Walker, with 

 the exception of the Sphaeridae, which were sent to Dr. Y. Sterki 

 for examination. 



GENERAL DISCUSSION. 



Physiography. 



The region studied (see map) lies in about 83° 15' longitude and 

 44° latitude, and is about 600 feet above sea-level. Sand 

 Point is four miles long, about one and one-half miles 

 wide at the base, and runs out almost due we.st into 

 Saginaw bay, tapering towards the point. North Island lies 

 about two miles south and one mile west of the end of Sand Point and 

 is approximately one mile long from e^st to west and one-fourth of a 

 mile wide ; Stony Island is somewhat over a mile still farther south and 

 west, is tadpole-shaped, and measures about one and one-fourth miles 

 from north to south, by one mile wide in the widest portion. Caseville 

 is .situated on the shore of the Bay, near the mouth of the Pigeon River, 

 about three miles north and east of the base of Sand Point ; the portion 

 of the Pigeon River studied is that running east from Caseville. Rush 

 Lake is about three miles long from the north-east to the south-west 

 corner and one mile wide, and lies about four miles east and one mile 

 north of Caseville. The portion of the Pinnebog River investigated is 

 about three miles due east of Ru.sh Lake. 



The shore of the Bay runs almost due north from Caseville for about 

 two miles to Oak Point, w'here it turns almost straight east so that 

 Rush Lake is but little over a mile from the shore of the Bay and lies 

 immediately behind the innermost sand dune. This strip of sand dunes 

 between Oak Point and Hat Point is about nine miles long by one wide. 

 It is broken along the Bay in three places by outcroppings of rocks, 

 viz., at Hat, Little Oak, and Oak Points. 



Sand Dunes. — The sand dunes were most thoroly studied on Sand 

 Point, but the strip along the coast between Oak and Hat Points was 

 also examined. The underlying rock in the latter region is Marshall 

 sandstone, with the exception of the western end; there and at Sand 

 Point the sand is underlaid by rocks of the Michigan or Lower Grand 

 Rapids Series, hydraulic Umestones, shales, gypsum, etc. Thruout 

 this entire region the rock is but a short distance below the surface so 

 that it outcrops in several places as mentioned above. (.A C. Lane* 

 states that it also should underlie Sand Point.) 



The shallow water off Little Oak and Hat Points most likely rejire- 



*Geol. Surv. of Mich., Vol. VII, part 2, page 106. 



