MAKYLAXD GEOLOGICAL SUEVEY 377 



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



Occurrence. — Choptank FoRMATiO]sr. Governor Eun, 2 miles south 

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

 Bridge. Calvert Formation. Wliite's Landing, near Friendship in 

 railway cutting. 



Collections. — 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, Rept. N. Car. Geol. Survey, p. 282, fig. 200. 

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

 Pecten Madisonitis Meek, 1804, Miocene Check List, Smith. Misc. Coll. (183), p. 4. 

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



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

 Pecten (Lyropecten) 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 striae : ribs elevated, rounded, with about three striae 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 fiat. 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 forms 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 ofi'shoot occurring in the St. Mary's formation probably, so that 

 the transitional forms found here render the discrimination of the two 



