12 THE CHICAGO ACADEMY OF SCIENCES. 
The subkingdom Mollusca is placed by most authors be- 
tween the worms (Vermes) and the crustaceans and insects 
(Arthropoda). By some it is considered an offshoot of the 
worms, and there is, in some respects, a strong similarity in the 
developmental history of the two subkingdoms. The group is 
somewhat related to the Vertebrata through the Cephalopoda, 
where the brain is protected by a cartilaginous covering and the 
eye is developed to a wonderful degree. The subkingdom is of 
quite recent date geologically, not appearing in any numbers 
until the Silurian period. From that time to the present they 
have greatly increased in numbers; there are at present known 
about 50,000 living and the same number of fossil species. 
Il. TOPOGRAPHY OF THE AREA.* 
The area in question is one of great interest to the malacol- 
ogist, since it has been very little written upon and almost noth- 
ing is known concerning the extent of its molluscan fauna. The 
area included is embraced in two drainage systems, one drain- 
ing into Lake Michigan through the lakes in the southern part 
of the region and the Chicago River, and one into the Mississippi 
River through the Desplaines, DuPage, Fox and Illinois rivers. 
The area includes all of Cook and DuPage counties, the nine 
northernmost townships of Will County, Illinois, and a small 
portion of the western part of Lake County,Indiana. The Jand 
and water area comprises about twenty-five hundred square 
miles, or eighteen hundred square miles of land surface. A 
remarkably low divide runs north and south between the north 
branch of the Chicago River and the Desplaines River, which at 
its lowest level is scarcely more than twelve feet above Lake 
Michigan. It is so low, in fact, that many times during high 
water the surface water from the Desplaines overflows its banks 
and runs into Lake Michigan through the north branch of the 
Chicago River. In this manner numerous species found in the 
Desplaines River are also found in the small ponds and creeks 
between it and the lake. 
In the southern portion of the region there are five small 
. lakes, lying partly in Cook County, Illinois, and partly in Lake 
County, Indiana. These are the Calumet (31%4x1% miles), 
George (21%4x,', miles), Wolf (3x1% miles), Berry (2x% miles) 
 iapop as extended account of the geology and topography of the area, see ‘‘ Th@ Flora of 
Cook County, Illinois, and a Part of Lake County, Indiana,’’ by Messrs. Higley and Raddin, 
and ‘' The Pleistocene Features and Deposits of the Chicago Area,’’ by Frank Leverett. 
