MAKVi.wi) (;i:<ti,o(!ic \i, srnvKY 101 



cut iiciir (licsapcakc Ucadi M. K'. slal imi). .\(,iii.\ |-"()U.\i \ rioN. TpixT 

 Marlboro, 2 iiiilus IjcIdw I'olomac Creek, 1 mile .>-<>ullica>l of Mason 

 Spri]i<?s, Clil'ton Beaeli, 1 mile northeast of I'ise-ataway. 



('(illnii()iis.—,]{)\\\\)^ Hopkins I'liiversity, iMaryland (leolo^fical Survey, 

 i'liiladelpliia Academy of Naliiial Sciences. 



Fan-lily PHOLADIDAH. 



Genus PHENACOMYA Dall. 



Phenacomya petrosa (C'oniad). 



i'lalc XXX, Figs. 4, 4a, 4b. 



Pholas petrosa Conrad, 1842, Proc. Nat. Inst., Bull, ii, \\ 1!>3, jjl. ii, fig. 4. 



Fholas petrosa Conrad, 184(), Amer. Jour. Sci., ser. ii, vnl. i, p. 21:^, pi. i, fig. 1. 



Pholas petrvm 11. C. Lea, 1848, Proc. Acad. Nat. Sci. Pliila., vol. iv, p. 104. 



Fholas petrosa Conrad, 1865, Amer. Jour. Concb., vol. i, p. 2. 



Pholas petrosa Conrad, 1866, Smith. Misc. Coll. (200), p. 9. 



Pholas {!) petrosa Clark, 189.5, Johns ITopkins Univ. Circ, vol. xv, p. 5; 



Pholas (?) petrosa Clark, 1896, Bull. 141, U, S. Geol. Survey, p. T3, pi. xv, figs. l«-lc. 



Phenacomtja petrosa Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, part iv, p. 82.3. 



Description.—-^' 0\?iie--ci.c\\\e\ anteriorly profoundly ventricose; radii 

 sharp and numerous; dorsal margin ol)liquely rectilinear from the sum- 

 mit of the umbo; base obliquely subrectilinear; jjosterior side produced, 

 cuneiform." Conrad, 1842. 



This form is apparently restricted to the limestone ledge, known as 

 Zone 5. 



Length, 85 mm.; height, 35 mm. 



Occurrence. — x^quia Foematiox. '" Piscataway "' (Conrad). Fort 

 Washington, Aquia Creek, Clifton Beach. 



Collections. — Johns Hopkins University, Maryland Geological Survey, 

 Philadelphia Academy of Xatural Sciences. 



Family GASTROCHAENIDAE. 



Genus GASTROCHAENA Spengler. 

 CtASTKOCHAEXA sp. 

 Plate XXX. Fig. 5. 



Gasirochaena sp. Clark, 1895, Johns Hopkins Univ. Circ, vol. xv, p. 5. 

 Oastrochaena sp. Clark, 1896, Bull. 141, U. S. Geol. Survey, p. 73, pi. xv, tig. 6. 



Casts of the tubes of Gastrocliaena are common in the indurated layer. 



Zone 5, of the Aquia Creek stage, and also occur at higher horizons. 

 11 



