570 Systematic Paleontology 



much deformed by scar of attachment; apical portion of shell spirally 

 coiled within the marginal outline of the shell ; hinge with ligamental 

 groove broad, deeply impressed, paralleled on the upper side by a rather 

 faintly developed, narrow, shallow groove, both grooves curved to conform 

 to spiral twist of shell ; posterior to the larger groove a broad, shallow, 

 pitted or striated depression ; surface of shell marked by thin, rather 

 prominent concentric, imbricating growth lamellae, with intermediate 

 fine growth lines ; costae either entirely absent or small, regularly arranged 

 costae present in proximity to beak and extending back from beak one-half 

 to three-fourtbs inch, or, in addition to the preceding, very faint irregu- 

 lar costae extending back to varying distances away from beak ; a more or 

 less clearly defined umbonal ridge extends from the beak backward, in a 

 curve conforming to the spiral twist of shell, to the lower posterior 

 margin, usually, however, becoming rounded and less clearly recognizable 

 toward the margin. Upper or right valve flat or slightly concave, operculi- 

 form, subcircular or subovate in outline, with a nearly flat, spiral twist, 

 the beak being well within the margin; beak depressed, not prominent, 

 this valve enclosed within and slightly depressed below the projecting 

 margin of the lower valve ; hinge with broad, deeply impressed ligamental 

 groove curved to conform to the spiral twist of shell, the upper margin 

 of the groove finely crenulated; posterior to the groove a striated pro- 

 tuberance occupies a position in apposition to the similarly striated 

 depression on the left valve; in proximity to the beak the surface is 

 marked by numerous fine, concentric growth lines, which away from the 

 beak toward the margins are produced into thin projecting lamellae, sepa- 

 rated by deep, narrow depressions." — Stephenson, 1914. 



The massive type of Exogyra ponderosa, which is so abundant and so 

 characteristic of certain horizons in the Gulf, has not been collected in 

 Maryland. The few individuals referred to this species are no heavier 

 than the E. costata, and differ from it only in the absence of all traces of 

 radial sculpture. 



Occurrence. — Matawan Fobmation. Old water-filled marl pit just 

 east of Post 236, Chesapeake and Delaware Canal, Delaware. 



