LamelUhranchiate Shells on the Hudson liiver Group. 139 



terminal. Cardiual line two thirds as long as the entire length of the 

 valve in its present shortened condition, and margined by a VQvy dis- 

 tinct and proportionally wide escutcheon like area of lanceolate form. 

 Margin of the shell arcuate from the anterior extremity- of the hinge, in- 

 cluding the anterior and basal bordei-s, the latter more sharply rounded 

 in the unnaturally shortened condition of the specimen, than the an 

 terior, while the posterior is somewhat more extended and again re- 

 curved to the extremit}' of the hinge just below the cardinal line. An- 

 terior en,d of the shell marked by a rather large and distinct lunule. 

 Surface marked only by concentric lines of growth, which are grouped 

 to form slight undulations on the surface of the shell. Were it 

 not for the distinctly formed lunule and escutcheon, we should 

 be inclined to place this species under the genus Leptodomus, McCoy; 

 but these characters readily distinguish it from the more typical forms 

 of that genus, and ally it strongl}' with Cuneamya, from which it differs 

 only in the extreme shortness of the shell. This has, however, been pro- 

 duced, to a considerable extent, hy compression in the longer axis of the 

 valves; and we should judge from the evidence furnished by the speci- 

 men, that this shortening has been equal to nearly one third of its pres- 

 ent length. The rounded form and great rotundity of the valves are 

 prominent features, and will serve to distinguish from any other 

 described form. 



Formation and locality: Found in the soft shales of the upper part 

 of the Hudson River Group, in Clinton Co., Ohio. 



Orthodesma mickleboroughi, n. sp. (Plate VI., fig, 7.) 



Shell of medium size, trapezoidal or sub-rhomboidal in outline, with 

 angular, ventricose valves, which are twice as long as high. Beaks 

 small, not very prominent, situated not more than one fifth of the entire 

 length of the shell from the anterior end, incurved, approximate and 

 sharply angular on the back. Cardinal line about three fourths as 

 long as the shell, and distinctly arcuate; anterior end short, the point 

 of greatest length being but little below the cardinal line, and the 

 margin below this point directed obliquel}' backward, at an angle of 

 from forty-five to fifty degrees with the cardinal line, to its junction 

 with the basal margin; basal line sub-parallel to the cardinal line, and 

 broadly sinuate at, or just posterior to, the middle of its length. Pos- 

 terior end obliquel}' truncate, rapidly receding from the extremity of 

 the hinge to the sharply rounded postero-basal angle. Posterior um- 

 bonal ridge prominent, angular throughout, but becoming more de- 

 cidedl}' so near the beak and at the postero-basal angle, causing a 

 downward projection of the shell margin; anterior umbonal ridge 

 rounded, but quite defined; cardinal slope abrupt and slightly concave, 



