94 PALAEONTOLOGY OF OHIO. 



as can be ascertained from several specimens and species examined, this 

 part of the shell being very similar in character to that of the genus 

 Anodonta. 



All the species yet fully determined as belonging to the genus are from 

 the Hudson River group, but we are of opinion that several forms found 

 in the Clinton group of New York will prove to have the same generic 

 features. 



Orthodesma recta (n. sp.). 



Plate 2, figs. 7, 8, 



Shell elongate, solen-like in outline, two and a half to three times as 

 long as wide, the cardinal and basal lines posterior to the beaks straight 

 and parallel; anterior end abruptly contracted beneath the beaks to one- 

 half the width of the body of the shell, somewhat extended and abruptly 

 rounded at the extremity ; posterior end as broad as the body of the shell, 

 oblicjuely rounded, longest at the postero-basal angle, and gently sloping 

 backwards to the extremity of the hinge line; beaks small and com- 

 pressed ; surface of the valves between the umbonal ridge and the an- 

 terior contracted portion depressed, forming a broad, shallow, and unde- 

 fined sulcus, strongest toward the beaks, and becoming obsolete or lost in 

 the general flattening of the shell before reaching the basal line, in 

 which it scarcely produces any perceptible feature. 



The surface of the valves is marked by irregular, concentric lines of 

 growth, and by several stronger undulations, which become somewhat 

 regular on the posterior slope for a short distance below the hinge line. 

 There are also appearances of one or two obscure secondary ridges on the 

 cardinal slope between the umbonal ridge and the cardinal margin, ex- 

 tending from near the beak to the posterior end of the shell. This latter 

 feature is extremely faint, and may be often not observable. 



The species is not readily confounded with any other found in the 

 same geological position, except, perhaps, 0. curvata, from which it may 

 be distinguished by the contraction of the basal line and the greater pos- 

 terior breadth of the shell in that species. It somewhat resembles 

 Orthodesma 2Mrallela^Orthonota parallela, Hall, Pal. N. Y., Vol. I., p. 299, 

 pi. 82, fig. 7, from the Hudson River group of New York, but ditfers in 

 the greater contraction of the anterior end, the broader beaks, the broad, 

 undefined depression of the median portion of the valves, and in the 

 form of the posterior end of the shell, which in that species is rounded, 



