FOSSILS OF THE HUDSON EIYER GROUP. 95 



while in this one it is obliquely truncate. The surface characters also 

 differ very materially in the two forms, that one having fine, even, regu- 

 lar, concentric lines over the anterior two-thirds of the valves. 



Formation and locality : In the soft shales of the upper part of the Hudson Biver 

 group, near "Waynesville, Ohio. The specimen figured was obtained, with several 

 others of different species, from Mr. Jesse Van Duser, of Waynesville, Ohio. 



Orthodesma curvata (n, sp.). 



Plate 2, fig. 6. 



Shell transversely elongate and narrow, the length equal to twice and 

 a half or three times as great as the greatest width ; cardinal and basal 

 margins sub-parallel, or very gradually diverging posteriorly to near the 

 posterior end ; cardinal line straight for one-third of the length of the 

 shell posterior to the beaks, beyond which point it gradually curves 

 downward to near the extremity, which is abruptly rounded, the longest 

 portion being below the center ; basal line gently concave for two thirds 

 of its length, but gradually rounding upwards at each end; anterior end 

 long, equal to about one-fifth, and in some individuals nearly one-fourth, 

 of the entire length of the shell, suddenly contracted beneath the beaks 

 to about one-third the width of the shell across the beaks ; beaks short 

 and broad, slightly enrolled, and obliquely sloping anteriorly ; body of 

 the shell moderately convex, with a faint, scarcely perceptible umbonal 

 ridge, anterior to which there is a broad, shallow depression crossing the 

 valves from the beaks to the basal line, occupying fully one-third of its 

 length. Anterior muscular scar strongly marked and of medium size, 

 situated very near to the anterior extremity ; posterior scar not observed. 



Surface of the valves marked by numerous concentric wrinkles, often 

 strongly marked on the cardinal slope near the cardinal border, and by 

 finer, irregular lines of growth. 



The species bears considerable resemblance to Modiolopsis nasuta, 

 Hall, from the Hudson River group of New York, and was, at first sight, 

 thought to be identical with that species; but, on comparison with 

 authentic specimens, it is found to have a much greater length posteri- 

 orly in proportion to the length of the anterior end, that one being about 

 twice as long posteriorly as anteriorly, while this species is from three to 

 four times as long. The length of the shell is also greater in proportion 

 to the height from the basal to the cardinal margin. 



Formation and locality : In the soft shales of the Hudson River group, near Waynes- 

 ville, Ohio. Collections of Columbia College and of Prof. Edward Orton. 



