298 



resembles 0. parallela, Hall (Pal. of New York, Vol. i., page 299, 

 pi. 82, fig. 7), but is much shorter anteriorly. 



Genus Nuctjla, Lam. 



NUCULA lOWENSIS (n. Sp.) 



Shell small, subovate or subtriangular in outline, very ventrlcose. 

 Beaks situated near the posterior (short) end, prominent and incurved. 

 Hinge plate bent abruptly beneath the beaks ; occupied by from five 

 to seven long narrow teeth on the long side, and by from three to five 

 smaller ones on the short side. Posterior end broadly rounded ; ante- 

 rior end prolonged, obtusely pointed ; basal margin strongly arcuate, 

 and the border of the shell thickened. 



Surface characters not determined. This species, like most of the 

 others, occurs in the condition of internal casts, and in some instances 

 the impressions of the exterior surface have not been preserved. 



This shell appears to be subject to considerable variation, at different 

 stages of growth ; young specimens often being distinctly triangular, 

 with the posterior end very short, and the basal margin but little 

 arched, while old specimens are subovate in form, and the posterior 

 end more prolonged. In one full-grown individual the muscular 

 impressions are very strongly marked, the anterior one being nearly 

 double the size of the posterior, and the basal portion of the shell 

 shows a great degree of thickening. 



Genus Leda, Schum. 

 Leda Barrisi (n. sp.) 



Shell elongate elliptical in outline ; the length twice as great as the 

 breadth ; valves very ventricose, most gibbous near the anterior end. 

 Beaks of medium size, situated about two-fifths of the entire length 

 from the anterior extremity; incurved, not prominent. Hinge fine 

 gently arcuate throughout its entire length; occupied by a large 

 number of small, curved teeth. Anterior extremity rounded, longest 

 below the middle ; basal margin gently arcuate ; posterior extremity 

 obliquely truncate, longest near the hinge line, with a slight emargi- 

 nation below. Umbonal slope slightly prominent, with a gentle 

 depression between it and the cardinal line. 



Surface marked by fine, closely arranged, equidistant, concentric 

 lines, which are distinctly undulated as they cross the umbonal slope 

 and the depression above it. Many of the internal casts preserve faint 

 impressions of the concentric lines, except near the basal margin, 

 where they are obscured by the thickening of the shell. A species 

 closely allied to this one occurs in the Chemung rocks of Ohio. 



