MARYLAXD GEOLOGICAL SURVEY 379 



sonius are broader and the radial threads finer and more numerous. 

 The best criterion, however, for their separation is found in the character 

 of the byssal ear. " In Jeffersoniiis it is sculptured with fine, uniform, 

 numerous threads, and the notch is shallow and leaves an inconspicuous 

 fascicle. In Madisonius the upper part of the ear is provided with com- 

 paratively few and coarse threads, and the notch is wide and deep with a 

 broad and well marked fascicle.^' Dall, loc. cit. 



Length, 130 mm.; width, 140 mm.; diameter, 25 mm., though often 

 foimd considerably larger. 



Occurrence. — St. Mary's Formatiok. St. Mary's Eiver. 



Collentions. — Maryland Geological Surv'ey, Johns Hopkins University. 



Pecten jeffersonius var. edgecombexsis (Conrad). 

 Plate C, Fig. 3. 



Pecten Edgecomensis Conrad, 1863, Proc. Acad. Nat. Sci. Phila., voL xiv, p. 391. 

 Pecten edgecomensis Courad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 581. 

 Pecten Jeffersonius var. edgecomensis Dall, 1898, Trans. Wagner Free Inst. Sci., vol. 

 iii, pt. iv, p. 723. 



Description. — " Suborbicular ; height not quite equal to the length ; 

 lower valve-ribs 16 to 17, prominent, but not elevated, square or convex- 

 depressed, not quite as wide as the intervening spaces, radiately lined 

 with finely squamose strise, most conspicuous towards the margins, inter- 

 stices of ribs carinated, in the middle squamose and finely striated; ears 

 with fine close unequal squamose radiating lines, the larger ones promi- 

 nent on the posterior side; margins of ligament pit carinated." Dall, 

 1898. 



The number of ribs in this variety varies from tv/elve to seventeen or 

 occasionally more. In the middle of the spaces between the ribs the fine 

 radial stri^ become somewhat larger. 



Length, 170 mm.; width, 185 mm.; diameter, 30 mm. 



Occurrence. — St. Mary's Formation. St. Mary's Eiver, Langley's 

 Bluff. 



Collections. — Maryland Geological Survey, U. S. National Museum. 



Pecten jeffersonius var, septenarius Say. 

 Plate C, Fig. 4. 



Pecten septenarius Say, 1834, Jour. Acad. Nat. Sci. Phila., vol. iv, 1st ser., p. 136, 

 pi. ix, fig. iii. 



