378 SYSTEMATIC PALEONTOLOGY 



species difficult at this horizon. For criteria for this discrimination see 

 remarks under P. jeffersonius. 



Length, 160 mm. ; width, 200 mm. ; diameter, 40 mm. 



Occurrence. — St. Mary's Formation. Cove Point, St. Mary's Eiver, 

 Langley's Bluff (Dall). Choptank Formation. Governor Run, 2 

 miles south of Governor Run, Flag Pond, Jones WTiarf, Cuckold Creek, 

 St. Leonard Creek, Turner, Pawpaw Point, Sand Hill, Dover Bridge, 

 Trappe Landing, Peach Blossom Creek, Cordova, Greensboro (Md. Geol. 

 Sur.) ; near Skipton (Dall). Calvert Formation. Fairhaven, Chesa- 

 peake Beach, 3 miles south of Chesapeake Beach, Plum Point, Truman's 

 ^^Hiarf, Church Hill, 3 miles west of Centerville, Reed's, White's Land- 

 ing, Wye Mills, Lyon's Creek, Magruder's Ferry. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 L^. S. National Museum, Philadelphia Academy of Natural Sciences. 



Pecten (Chlamys) jeffersonius Say. 

 Plate C, Fig. 2. 



Pecten Jeffersonius Say, 1824, Jour. Acad. Nat. Sci. Pblla., vol. iv, 1st ser., p. 133, 



pi. ix, fig. 1. 

 Pecten Jeffersonms Conrad, 1840, Fossils of the Medial Tertiarj', x^- 40, pi. xxii, fig. 1. 

 Pecten Jeffersonius Emmons, 1858, Rept. N. Car. Geol. Survey, p. 'J82, fig. 199. 

 Pecten Jeffersonius Conra,d, 1863, Proe. Acad. Nat. Sci. Phila., vol. xiv, p. 581. 

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

 Pecten (Lyropecien) Jeffersonius Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, 



pt. iv, p. 722. 



Descriftion. — " Subequivalve, with from nine to eleven striated ribs. 



" Shell rounded, convex, not quite equivalved, one of the valves being 

 a little more convex than the other; the whole surface covered with 

 ap^oroximate scaly strise : ribs elevated, rounded, with six or seven striae 

 on the back of each ; intervening grooves profound : ears equal ; sinus of 

 the ear of the superior valve, not profound, being barely one-eighth part 

 of the length of the ear : within with broad rounded flattened ribs." Say, 

 182-1. 



This species is very probably a descendant of P. madisonius and is at 

 times hard to distinguish from it. In general, jeffersonius is the more 

 convex, the upper and lower valves being nearly equi-convex; while in 

 madisonius the upper valve i? flatter thau the lower. The ribs of jeffer- 



