MARYLAND GEOLOGICAL SURVEY 635 



Occurrence. MONMOUTH FORMATION. Brooks estate near Seat Pleas- 

 ant, Prince George's County. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences. 



Outside Distribution. Manasquan Formation. ? New Jersey. 



Superfamily ENSIPHONACEA 

 Family POROMYACIDAE 



Genus LIOPISTHA Meek 

 [Check List Invt. Foss. N. A., Cret. and Jur., 1864, p. 32] 



Type. Cardium elegantulum Eoemer. 



" Shell equivalve, inequilateral, transversely subovate, being usually 

 narrower, more compressed, and often subrostrate behind, and ventricose 

 in the central and umbonal regions, nearly always extremely thin; 

 extremities rounded in outline, the posterior side usually a little gaping ; 

 surface granular, and varying, according to the sections and species, from 

 radiately costate on the flanks and front of the valves, to strongly undu- 

 late concentrically, with only a few impressed, radiating lines on the 

 middle, or rarely nearly smooth, concentrically striate, or furrowed, with 

 obsolescent radiating striae ; dorsal margins generally inflected so as to 

 form a sort of false area along its entire length ; hinge with two promi- 

 nent cardinal teeth projecting out at right angles from close up under 

 the hinge line, beneath the beak of the right valve (the posterior tooth 

 being larger and compressed, and the anterior pointed), and one promi- 

 nent and one rudimentary cardinal tooth under that of the left; lateral 

 teeth, none; ligament external; fulcra short and erect. Pallial line 

 unknown. 



" Liopistha Meek (typical). Shell transversely subovate, ornamented, 

 excepting on the posterior dorsal portions of the valves, by regular, simple, 

 well-defined, sometimes subcrenate, radiating costa?." Meek, 1876. 1 



The genus is restricted in its distribution to the Cretaceous. 



A. Secondary radial sculpture not developed Liopistha protexta 



B. Secondary radial sculpture developed Liopistha alternata 



Etymology: Xeios, smooth; S-n-iffOei', behind. 

 1 U. S. Geol. Survey Terr., vol. ix, p. 227. 



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