316 MEMOIRS OF THE CARNEGIE MUSEUM. 



certain parts of the shell. This color is so eminently characteristic, that even 

 laymen have noticed it, and people living on the banks of the Susquehanna and 

 Delaware properly distinguish this species as the "yellow clam." 



The variation of this species occurs chiefly in the external shape (more or less 

 elongate) and the development of the rays. Moreover the yellow of the ground- 

 color varies slightly, being lighter or more intense, often inclining toward reddish 

 bro\Mi in parts of the shell. 



Localities in Pennsylvania represented in the Carnegie Museum: 



Delaware River, Taylorsville (C. H. Conner) and Yardley, Bucks Co. ; Shawnee, Monroe Co.-"* 

 Susquehanna River, York Furnace and York Haven, York Co.; Duncannon, Perry Co.; Selinsgrove, 



Snyder Co. 

 Conewago Creek, York Haven, York Co. -"^ 



Juniata River, Juniata Bridge, Perry Co.; Lewistown, MifBin Co. 

 Frankstown Branch, Juniata River, Huntingdon, Huntingdon Co. (D. A. Atkinson). 

 Raystown Branch, Juniata River, Ardenheim, Huntingdon Co. 

 West Branch Susquehanna River, Williamsport, Lycoming Co. (D. A. Atkinson). 

 North Branch Susquehanna River, Tunkhannock, Wyoming Co. 

 Chemung River, South Waverly, Bradford Co. 



Other localities represented in the Carnegie Museum: 



Connecticut River (Hartman collection). 



Delaware River, Newbold, Gloucester Co., New Jersey (C. H. Conner). 



Potomac River, Cabin John, Montgomery Co., Maryland (J. D. Haseman); Mount Vernon, Fairfax 



Co., Virginia (Juny collection). 

 Savannah River (Hartman collection).^"' 



Distribution and Ecology in Pennsylvania (See fig. 31): This species belongs 

 to the Delaware and Susquehanna drainages in Pennsylvania, but has not been 

 found in the small tributaries of the Potomac in our state. In the Delaware River 

 it goes up to above the Delaware Water Gap, but its scarcity just above the gap 

 indicates, that at this point it approaches the upper limit of its range. In the 

 Susquehanna it is practically everywhere, ascending at least to Williamsport, 

 Lycoming Co., in the West branch, and to the New York state line in the North 



'"* Shawnee is immediately above the Delaware Water Gap : only one specimen was found here and 

 no dead shells were Ijdng around. 



'"^ In the lowermost part only of this creek, where there is at times water from the Susquehanna. 



-"' A large, thick-shelled specimen, labeled rosaceus Conrad, agrees well with Conrad's description, 

 but is larger, and has only a faint blush of pink on the nacre. In other respects it agrees likewise with 

 cariosus, and, although discolored, the epidermis shows remnants of the gloss. If locality and identifica- 

 tion are correct, rosaceus is a form of cariosus, not of ochraceus (of which Simpson makes it a synonym). 

 Whether this form is different from cariosus (as variety or species) cannot be decideil from a single indi- 

 vidual. 



