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 w^ould naturally consider them distinct species. 



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

 L. phalacm is one of the intermediate forms with the strife partially obso- 

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

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

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

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

 tained as a variety. 



Length, 10.5 mm.; height, (5.1 mm.; diameter, 2.5 mm. 



Occurrence. — Ciioptank 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. National ]\Iuseum, Philadelphia Academy of Natural Sciences. 



Leda liciata var. amydra Dall. 

 Plate CVII, Figs. 9, 10. 



Leda amydra Dall, 1808, Trans. Warner Free Inst. Sci., vol. iii, pt. iv, pp. 591, 592. 



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

 ately convex, with an evenly arcuate l)ase, 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 ehon- 

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

 side of it; the rostrum is short, rounded, and witliout 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 members 

 of the series of which it is an end member, l)y its smooth, jiolished surface. 



