THE NAUTILUS. 51 



able weight to a theory that P. torrei is more closely related to 

 P. constellata than to P. stellata. 



EXPLANATION OF FIGURES, PLATE 4. 



All figures were drawn with the aid of a camera lucida. 

 Fig. 1. P. torrei: c. central; i, iv laterals; u. uncini. 

 Fig. 2. Uncini 1 and 2, 38 and 39, 45, 55, 70, 90, 102 (the 

 outermost) . 



Fig. 3. Central tooth of P. constellata. 

 Fig. 4. Central tooth of P. stellata. 

 Figs. 5, 6, shell of P. torrei. 



LAMPSILTS VENTRICOSA COHONGOKONTA IN THE POTOMAC VALLEY. 



BY WM. B. MARSHALL. 



In the NAUTILUS for October, 1917, I recorded the finding of 

 two valves (belonging to the same individual) of this shell by 

 Manly D. Barber in the Potomac River, at Great Falls, Mary- 

 land, about eighteen miles above Washington, D. C. Dr. Ort- 

 mann had already recorded the finding of a single specimen as 

 far south as the Shenandoah River, at Harper's Ferry, W. Va., 

 some fifty miles above Great Falls, and the finding of others at 

 places farther up the river. 



On July 7, 1918, Dr. C. Wythe Cooke of the U, S. Geologi- 

 cal Survey, found a superb specimen living in a sandy pass at 

 Midriver Island, which is only about a mile and a half above 

 the Falls. 



On July 28, 1918, he and I donned bathing suits and made 

 a careful examination of the spot in the hope of finding more 

 specimens, and especially the very young. For two or three 

 hours we explored the sand and the mud beneath with our 

 fingers and toes and passed quantities of sand and mud through 

 our hands and through a fine mesh sieve. The spot thus in- 

 vestigated was about 20 feet wide and 75 feet long. Our efforts 

 were rewarded by the finding of four specimens of cohongoronta, 

 the smallest having a length of 40 mm., the largest a length of 



