12 PROC'ERDINGS OK THK ACADEMY OF [1894. 



each valley supplying a creek tributary to the Potomac. The valleys 

 are mostly eroded anticlines, so that a great variety of strata are ex- 

 posed ; but sandy shales and shaly sandstones predominate. 



The Potomac crosses this series of parallel ridges nearly at a right 

 angle. It is everywhere shallow and rapids are frequent. At Cum- 

 berland it is dammed, and all the water which is not used by the city 

 water-works feeds the Chesapeake and Ohio Canal, which has its 

 western terminus here. Above the backwater from this dam it is a 

 shallow rapid stream from forty to sixty feet wide, with a rocky, or in 

 places, gravelly bed ; and the low banks are fringed with graceful 

 maple, willow and sycamore trees, often festooned with grape vines, 

 and in places mingled with oak and locust. The water here is of a 

 dark sepia tint, being stained by the spruce sawdust from saw-mills 

 upon its upper course, and probably also by decaying laurel and 

 bay leaves, for the region above is densely wooded. No mollusk life 

 whatever could be found in this brown water, but minnows and tad- 

 poles were seen. Below the dam at Cumberland the wide river-bed 

 is dry, except for occasional pools, in which a few Planorbis blcarina- 

 tus and Amnicola limosa live. 



Except in times of high water therefore, the lower course of the 

 river is an entirely distinct stream from this upper reach. About 

 ten miles below Cumberland the river has about the same volume as 

 the stream above the influence of the Cumberland dam, and the 

 water is clear. 



Wills Creek, which flows through the city of Cumlierland, is a 

 swift stream with a rocky bed. The water is of crystal transparence, 

 but no snails except Planorbis bicarinalHs were found in it. Evitt's 

 Creek, which drains the valley next eastward, contains Ancidosa in 

 abundance. 



Family SELENITID^. 



This family differs from Zonituhr \n having the teeth of the radula 

 all "aculeate," and in lacking pedal grooves above the foot-edges. 



Genus SELENITES Fischer. 



Selenites concavus Say. 



Cumberland, Allegheny County, Md., 64,679. Morgan County, 

 W. Va., opposite Hancock, 64,678. It has been taken by Mr. 

 W. Stone at York Furnace, York County, Pa., and by Mr. C. W. 



