MAEYLAND GEOLOGICAL SUKYEY 371 



Description. — " Obliquely obovate, thin and fragile, inflated ; with 

 prominent radiating lines, distant towards the anterior margin; anterior 

 margin angulated at base of the ear, truncated or slightly concave below, 

 and abruptly rounded where it joins the basal margin; ears small. 

 . . ." Conrad, 1841. 



This species has very rarely been collected. It seems confined to a 

 thin band found here and there at an elevation of three or four feet above 

 tide in the cliff just south of Plum Point. It is very fragile and diffi- 

 cult to obtain entire. 



Height, 25 mm. ; width, 21 mm. ; diameter, 7 mm. 



Occurrence. — Calvert Fobmation. Plum Point. 



Collection. — Maryland Geological Survey. 



Family SPONDYLID/E. 



Genus PLICATULA Lamarck. 

 Plicatula densata Conrad. 

 Plate XCVIII, Figs. 7, 8, 9. 



Plicatula densata Conrad, 18-13, Proc. Acad. Nat. Sci. Phila., vol. i, p. 311. 

 Plicatula densata Conrad, 1845, Fossils of the Medial Tertiary, p. 75, pi. xliii, tig. 6. 

 Plicatula densata Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 583. 

 Plicatula densata Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 4. 

 Plicatula densata Whitfield, 1894, Men. xxiv, U. S. Geol. Survey, p. 35, pi. v, figs. 



3-8. 

 Plicatula densata Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, pt. iv, p. 763. 



Description. — "Ovate, thick, profoundly and irregularly plicated; 

 inferior valve ventricose; ribs acute, with arched spiniform scales; car- 

 dinal teeth large, curved, laterally striated, crenulated on the margins; 

 larger cardinal tooth in each valve slightly bifid, broad ; muscular impres- 

 sion prominent. . . . The valves have about ten folds, and the lower 

 valve closely resembles a variety of Ostrea Virginiana." Conrad, 1843. 



This species is distinguished from the P. marginata Say, by its broader, 

 rounder, flatter form, more irregular and less prominent as well as finer 

 plications and greater tendency to lateral curvature of the beaks. 



Length, 37 mm.; width, 31 mm.; diameter, 6 mm. 



Occurrence. — Calvert Formation. Church Hill. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



