MARYLAND GEOLOGICAL SURVEY 305 



side narrowed ; umbo very oblique, prominent ; posterior margin arcuate ; 

 inner margin deeply crenulated. 



" This shell has probably been confounded with V. tridacnoides , but it 

 is much thinner, not undulate on the disk, and the cardinal teeth are much 

 less robust. Its narrowed and compressed anterior side will distinguish it 

 from the other fossil species, and its ribs from the recent Y. ■merccnaria. 

 Young shells are compressed or plano-convex. The disks are generally 

 worn, showing the radiating striae common to all these large fossil species 

 when the surface becomes decomposed. It is named in compliment to my 

 scientific friend. Dr. William Eiley of Baltimore." Conrad, 1838. 



This species as found at Plum Point is not notably thick. The hinge 

 area is narrow, and the teeth rather small. The umbo can scarcely be 

 considered prominent. The great proportionate length of the shell dis- 

 tinguishes this species readily from the others of the Miocene. None of 

 the Plum Point specimens show the great thickening or undulations on 

 the disk so characteristic of the tridacnoides as found at numerous Virginia 

 localities. For this reason, and because the Virginia beds in which 

 thickened shells are found are much higher stratigraphically in the 

 Miocene than the Plum Point beds the writer prefers to retain the name 

 rileyi for a distinct species. 



Length, 115 mm.; height, 80 mm.; diameter, 23 mm. 



Occurrence. — Calvert Formation. Plum Point. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



Venus mercenaria Linne. 

 Plate LXXVIII, Figs. 1, 2. 



Venus mercenaria Linne, 1758, Syst. Nat., Edit, x, p. (J8(j. 



Venus mercenaria Tuomey and Holmes, 1856, Pleiocene Fossils of South Carolina, 



p. 81, pi. xxi, fig. 6. 

 Venus mercenaria Emmons, 1858, Kept. N. Car. Geol. Survey, p. 392. 

 Mercenaria violacea Holmes, 1858, Post-Pleiocene Fossils of South Carolina, p. 33, 



pi. vi, flg. 11. 

 Mercenaria mercenaria Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 574. 

 Venus mercenaria Gould (Binney's), 1870, Invert. Mass., p. 133, fig. 1:45. 

 Mercenaria concellata Whitfield, 1894, Mon. xxiv, U. S. Geol. Survey, p. 68, pi. xii, 



figs. 2-3. 

 Venus mercenaria Dall, 1903, Trans. Wagner Free Inst. Sci., vol. iii, pt. vi, p. loll. 



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