52 THE NAUTILUS. 



A case parallel to this has now come to light : the presence, in 

 the Potomac system, of a western mussel, Lampsilis ventricosa 

 (Barnes). This is a form common, for instance, in the Ohio drain- 

 age in western Pennsylvania and West Virginia, but it is not found 

 in any of the Atlantic river systems investigated by the writer (Dela- 

 ware, Susquehanna, James). The following records are at hand : 



September 4, 1909. Potomac River, Hancock, "Washington Co., 

 Md. Here it was iu the smaller branch of the river in great num- 

 bers and in all sizes, from 41 mm. long upward. About two dozen 

 were taken. 



May 9, 1911. South Branch Potomac River, Southbranch, 

 Hampshire Co., W. Va. About a dozen were found within the first 

 mile of the South Branch above its junction with the North Branch. 

 All were of medium size. 



August 16, 1911. Shenandoah River, Harper's Ferry, Jefferson 

 Co., W. Va. A single male was found, below medium size. 



May 6, 1912. South Branch Potomac River, Romney, Hamp- 

 shire Co., "W. Va. About a dozen were found in a very small 

 branch of the river. Some were rather large, but most of them 

 were quite small (smallest 20 mm. long). These small ones were 

 found all together in fine gravel at the head of a riffle, in shallow 

 water, attached to small pebbles by their byssus. 



In other parts of the Potomac drainage, thoroughly investigated, 

 this species was not found, and this is especially true for the head- 

 waters of the Shenandoah River and the northern tributaries of the 

 master stream (Antietam, Conococheague and Great Tonoloway 

 Creeks in Pennsylvania and Wills Creek in Maryland). 



It is probable that this species will turn up elsewhere in the 

 Potomac. The localities known at present are all to the West of the 

 Blue Ridge Mountain, that is to say, within the Great Allegheny 

 Valley and the Allegheny Mountains. It should be noted, that its 

 representative Atlantic form, Lampsilis cariosa (Say), which is so 

 common in the Delaware and Susquehanna, is not found in this 

 region, but is present farther below in the Potomac : I have speci- 

 mens from Cabin John, Montgomery Co., Md. 



There is no question that we have to deal with a form of Lamp- 

 silis ventricosa. It is true that the Potomac form represents a pecu- 

 liar type in so far, as all the specimens collected are rather uniform 

 in shape, but the same shape is frequently met with in western speci- 



