396 SYSTEMATIC PALEONTOLOGY 



the umbonal slope and partially or entirely obsolete near the margin. In 

 other specimens a larger and larger portion of the surface in this way 

 becomes smooth^ until they finally grade over into perfectly smooth pol- 

 ished forms. One having but a few valves belonging to different portions 

 of the series would naturally consider them distinct species. 



Leda acryhia is a typical L. liciata and stands at one end of the series. 

 L. phalacra is one of the intermediate forms with the striae partially obso- 

 lete, while L. amydm is the smooth, polished variety forming the other 

 end member of the series. It has not been practicable to separate the in- 

 termediate forms from the liciata. They are accordingly grouped to- 

 gether, while the smooth polished end member, L. amydra, has been re- 

 tained as a variety. 



Length, 10.5 mm.; height, 6.1 mm.; diameter, 2.5 mm. 



Occurrence.— Choptank Formation. Greensboro. Calvert For- 

 mation. Chesapeake Beach, 3 miles south of Chesapeake Beacli, Plum 

 Point, Truman's Wharf. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 U. S. ISTational Museum, Philadelphia Academy of Natural Sciences. 



Leda liciata var. amydra Dall. 

 Plate CVII, Figs. 9, 10. 



Leda ami/dra Dall, 1898, Trans. Waoner Free Inst. Sci., vol. iii, pt. iv, pp. 591, 592. 



Description. — " Shell small, smooth, polished, subequilateral, moder- 

 ately convex, with an evenly arcuate base, no lunule, and the escutcheon 

 small, narrow, excavated, bounded outside by a raised line beyond which 

 is a second furrow extending nearly to the end of the rostrum ; tiie chon- 

 drophore is small and deep-seated with about a dozen small teeth on each 

 side of it; the rostrum is short, rounded, and without any internal par- 

 tition .... 



"This shell is remarkably like a small Leda from the Claiborne sands 

 which I have without a name, but is more rounded behind. More mate- 

 rial is needed to establish its exact relations." Dall, 1898. 



Dr. Ball's description was from a single valve found at Plum Point. 

 It is but a variety of L. liciata and distinguished from the other iiiemlDers 

 of the series of which it is an end member, by its smooth, poli.^lied surface. 



