26 PROCEEDINGS OF THE ACADEMY OF [1894. 



Genus BYTHINELLA, Auct. 

 B. nickliniana Lea. 



Conecocheague Kiver, west of Hagerstovvu, Md., 64, 757. 



Family PLEUROCERID^E. 



Genus ANCULOSA Say. 

 A. carinata Brug. 



This species is the characteristic mollusk of perennial streams in the 

 Chesapeake Bay river-system. It has been found only in waters of 

 this drainage, being unknown in the adjacent Delaware system on 

 the north-east, in the Ohio system on the west, and in streams empty- 

 ing into the Atlantic to the south of Chesapeake Bay. 



Aiiculosa being essentially a ti-ans-Alleghenian genus, we may 

 with considerable confidence surmise that the ancestors of A. carin- 

 ata were introduced into the head waters of the Potomac from some 

 creek of the Ohio system, and from this colony spread throughout the 

 Chesapeake drainage. The fact that it inhabits the James River 

 and other Virginian streams which are now isolated from the Poto- 

 mac and Susquehanna by a long stretch of salt water, indicates that 

 since the introduction of Ancidosa, and the differention of the species 

 carinata, the Chesapeake region has been much more elevated than 

 it is at present. Ancalosa can endure neither salt nor slow- flowing 

 water; and it must have spread to these various streams at a time 

 when the united waters of the Susquehanna, Potomac, Ra})pahan- 

 nock and James Rivers flowed in one mighty stream to the Atlantic. 

 It may be suggested that the distribution has lieen accomplished 

 overland; but tliis is highly improbable; for if so, why is the species 

 so strictly limited to the Chesapeake system? Why has it not invaded 

 the Delaware-Schuylkill drainage, which otters equally favorable 

 stations, and is separated by but a few miles from the eastern tribu- 

 taries of the Susquehanna? We must conclude that Strepoinafldn' 

 require as a rule actual water counnunieation for their spread from 

 stream to stream. The exceptions are probably rare and unimpor- 

 tant. 



The presence of this species in the headwaters of the Roanoke 

 River at the hamlet of Lafayette, Montgomery Co., Va. , is readily 

 accounted for by the close proximity of the creeks forming the head 

 of the James and those flowing into the Roanoke. Some time a* 



