MAKYLAND GEOLOGICAL SUKVEY 377 



Length, 69 mm.; width, 67 mm.; diameter, 11 mm. 



Occurrence. — Choptank Formation. Governor Eun, 2 miles south 

 of Governor Eim, Flag Pond, St. Leonard Creek, Jones Wharf, Dover 

 Bridge. Calvert Forj^iation. Wliite's Landing, near Friendship in 

 railway cutting. 



CoZZec^io^Js.— Maryland Geological Survey, Johns Hopkins University, 

 Philadelphia Academy of Natural Sciences. 



Section LYROPECTEN Conrad. 



Pecten (Chlamys) madisonius Say. 

 Plate C, Fig. 1. 



Pecten Madisonius Say, 1824, Jour. Acad. Nat. Sci. Phila., voL iv, 1st ser., p. 134. 

 Pecten Madisonius Conrad, 1840, Fossils of the Medial Tertiary, p. 48, pi. xxiv, fig. 1. 

 Pecten Madisonius Emmons, 1858, Kept. N. Car. Geol. Survey, p. 283, fig. 200. 

 Pecten Madisonius Conrad, 1868, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 581. 

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

 Pecten 3fadlsonius Whitfield, 1894, Mon. xxiv, U. S. Geol. Survey, p. 30, pi. iv, 



figs. 1-5 ; pi. ii, fig. 8. 

 Pecten [Lyropeeten) Madisonius Dall, 1898, Trans. Wagner Free Inst. Sci., voL iii, 



pt. iv, p. 724. 



Description. — " Much compressed, with about sixteen striated ribs. 



''Shell rounded, much compressed; the whole surface covered with 

 scaly stri^ : rihs elevated, rounded, with about three stri^ on the back of 

 each; intervening grooves rather profound: ears equal, sinus of the ear 

 of the superior valve profound, extending at least one-third of the length 

 of the ear." Say, 1824. 



The ribs are usually about sixteen or seventeen, but occasionally as few 

 as twelve; lower valve convex, upper one nearly flat. The young from 

 the Calvert formation often have but one prominent elevated spinose 

 line on the back of each rib, with a faintly marked one on either side 

 especially near the margin of the shell. A series of intermediate speci- 

 mens from here shows a close relationship with P. coccymelus found at 

 the same horizon. Another series of intermediate formes from the Chop- 

 tank formation suggests a relationship to the P. marylandicus found at 

 that horizon. From P. madisonius is probably descended P. jeffersonius, 

 the ofl'shoot occurring in the St. Mary's formation probably, so that 

 the transitional forms found here render the discrimination of the two 



