LAMELLIBRANCHIATA. 313 



Test thin, marked by moderately strong concentric striae, which are raised 

 into varices or concentric undulations at irregular intervals. 



Anterior muscular impression just within the anterior border, above the 

 longitudinal center of the shell. 



The three specimens figured measure respectively 45, 37 and 34 mm. in 

 length, and 35, 28 and 27 mm. in height. 



This species differs from any other described, in the stronger curvature of the 

 basal margin, bringing the termination of the umbonal ridge higher, and giving 

 a proportionally narrower cardinal slope and posterior end ; in other respects, 

 it is very similar to the M. tenuistriatus of the Hamilton group. 



Formation and locality. In the Waverly sandstones of Licking county, 0. 



NUCULA, Lamarck. 1799. 



NUCULA NIOTICA. 



PLATE XI.V, FIGS. 1,2. 



Nucala niotica. Hall and Whitfield. Twenty-fourth Ann. Rep. N. Y. State Mus. Nat. Hist., p. 190. 1872. 

 „ .< .. " Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 45, 



figs. 1, 2. 1883. 



Shell small, obtusely sub-cuneiform ; length a little greater than the height ; 

 basal margin regularly curving, rounded posteriorly ; cardinal margin very 

 oblique ; anterior margin vertically truncate. 



Valves very gibbous. 



Beaks anterior, incurved. Umbo prominent. Umbonal slope very gibbous. 



Test thick in the upper part, marked by fine, even, concentric strise, which 

 are aggregated into fascicles of growth. 



The internal casts show strong anterior and posterior muscular impres- 

 sions, and three distinct umbonal muscles ; there are seven or more pos- 

 terior, and five anterior teeth in a specimen of medium size. 



An internal mould has a length of 17 mm., and a height of 13 mm. A 

 specimen preserving the test has a length of 18 mm., and a height of 16 mm. 



