Maryland Geological Survey 515 



NUCULA MICROSTRIATA n. sp. 



Plate XIX, Fig. 7 



Description. — Shell small, very thin and fragile, ovate in outline, 

 moderately inflated in the umbonal region, compressed toward the mar- 

 gins; umbones rather inconspicuous, their apices acute, opisthogyrate, 

 slightly posterior in position ; lunule much produced, ill-defined by reason 

 of the flattening of the valves; escutcheon quite deeply impressed, 

 elongate-cordate in outline ; dorsal margins oblique, diverging from the 

 apex at an angle of about 100° ; posterior dorsal slope steeper than the 

 anterior; anterior extremity broadly rounded, merging more gradually 

 into the dorsal margin than into the ventral; posterior dorsal margin 

 apparently produced until it joins the upcurved base line; ventral margin 

 very broadly arcuate; entire external surface sculptured with micro- 

 scopically fine and crowded concentric stride ; hinge characters somewhat 

 obscured; teeth, short serrate, arranged in two discrete series separated 

 by an oblique chondrophore ; anterior series numbering about twenty, the 

 posterior less than half as man}' ; characters of adductor scars and pallial 

 line not known. 



Dimensions. — Altitude 6 mm. ; latitude 8 mm. ; semi-diameter 2.5 mm. 



This species is well characterized by the small size, thin shell and very 

 finely and closely striated external surface. 



Occurrence. — Monmouth Formation. Brightseat, Prince George's 

 County. 



Collection. — Maryland Geological Survey. 



Family LEDIDAE 

 Genus LEDA Schumacher 

 [Essai, 1817, pp. 55, 172] 



Type. — Leda rostrata Gmelin. 



Shell solid, porcellaneous, transversely elongate, rounded anteriorly, 

 more or less rostrate posteriorly; beaks proximate, often tumid, feebly 

 opisthogyrate ; exterior surface concentrically sculptured ; hinge armature 



Etymology: Leda, the mother of Castor and Pollux. 



