MAEYLAND GEOLOGICVL 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 Jeffersonvus it is sculptured with fine, uniform, 

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

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

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

 broad and well marked fasciole." Dall, loc. cit. 



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

 found considerably larger. 



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



Collections. — Maryland Geological Survey, Johns Hopkins University. 



Pecten jeffersonius var. edgecombensis (Conrad). 

 Plate C, Fig. 3. 



Pecten Edgecomensis Conrad, 1862, 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. 722. 



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 striae, 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 strise become somewhat larger. 



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



Occurrence. — St. Maey^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, 1824, Jour. Acad. Nat. Sci. Phila., vol. iv, 1st sen, p. 136, 

 pi. ix, fig. iii. 



