MAEYLAND GEOLOGICAL SURVEY 375 



Gradient forms show a close genetic relationship with P. madisonius 

 as found in the Calvert formation. 



Length, 30 jnm. ; width, 25 mm. ; diameter, 5 mm. 



Occurrence. — Calvert Formation. Plum Point, Chesapeake Beach, 

 3 miles south of Chesapeake Beach. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 U. S. National Museum. 



Section NODIPECTEN Dall. 



Pecten (Chlamys) rogersi Conrad. 

 Plate XCIX, Fig. 4. 



Pecten Rogersii Conrad, 1834, Jour. Acad. Nat. Sci. Phila., vol. vii, 1st ser., p. 151. 

 Pecten Rogersii Conrad, 1840, Fossils of the Medial Tertiary, p. 45, pi. xxi, fig. 9. 

 Pecten Rogersi Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 581. 

 Pecten Rogersi Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 4. 

 Pecten [Nodipecten) Rogersi Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, pt. iv, 

 p. 730. 



Description. — " Shell ovate, compressed ; with four very large and 

 broad convex ribs and numerous radiating lines; ears small. Length 

 and height, one inch and one-eighth." Conrad, 1834. 



" Shell with four large and two smaller lateral simple ribs ; internally 

 lirate; submargins narrow, minutely scabrous, not radiated; the rest of 

 the disk entirely covered with fine, squared, elevated, minutely scaly 

 radial threads; ears subequal, finely radiated; sinus well-marked; cte- 

 nolium and cardinal crura developed." Dall, 1898. 



Length, about 13 mm., specimen broken and young. 



Occurrence. — Choptank Formation ( ?). Near Skipton. 



Collection. — ^U. S. National Museum. 



Section PLACOPECTEN Verrill. 



Pecten (Chlamys) clintonius Say. 

 Plate XCIX, Fig. 5. 



Pecten Clintonius Say, 1824, Jour. Acad. Nat. Sci. Phila., vol. iv, 1st ser., p. 135, 



pi. ix, fig. 2. 

 Fiseten Clintonius Conrad, 1840, Fossils of the Medial Tertiary, p. 47, pi. xxiii, fig. 1. 

 Pecten Clintonius Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 581. 

 Pecten Clintonensis Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 5. 

 Pecten (Placopecten) Clintonius Dall, 1S9S, Trans. Wagner Free Inst. Sci., vol. iii, 



pt. iv, p. 725. 



