NAPLES FAUNA IN WESTERN NEW YORK, PART 2 289 



with rather prominent umbo, and closely appressed, incurved beak. Cardi- 

 nal area short, triangular, but inequilateral, and divided unequally by the 

 beak. Surface quite convex, maximum convexity being attained near the 

 middle of the shell, and the curvature thence becoming slightly concave on 

 the anterior and posterior slopes. Outline subcircular to transversely oval. 



The sculpture consists of fine radial lines, from 125 to 150 in number, 

 which extend over the entire surface to the hinge line. These are at first 

 round and filiform, but become flattened outward and increase by unequal 

 division, so that on the margin these radii alternate or vary irregularly in 

 size. The intervening grooves are very narrow. At the cardinal slopes 

 the lines are larger on the posterior slope, and their extremities inosculate 

 on opposite valves. Over the median portion of the shell these radial lines 

 often show a decided backward curve parallel with the oblique line of 

 greatest convexity from the beak to the ventral margin. In occasional 

 instances duplication of the radial striae begins early and results in 

 exceptionally fine lineation of the pallial region of the shell. 



The surface also sometimes bears low, somewhat irregular concentric 

 folds which seem to be largely due to compression in the shales, as rotund 

 specimens are free from them. 



We have already noted the remarkable similarity in ornamental 

 characters between this shell and Paraptyx Ontario, which conceals 

 the distinction between the two when the cardinal parts are not retained. 

 In O. clarkei the hinge line is very short, and the cardinal angles 

 rounded closely about the beak, while the umbonal striae radiate to the 

 posterior edge. 



Dimensions. In average specimens the length is 20 mm, hight 18 

 mm. Large examples attain a length of 30 mm and hight of 25 mm, while 

 small individuals very much below these dimensions are common. 



Habitat. Genesee province ; Naples subprovince. This species is 

 common in the shales of the Naples beds in Ontario and Livingston 

 counties, but has not been observed west of the Genesee river. Beushau- 

 sen's specimen is from the lower Upper Devonic at Oberscheld. 



