NAPLES FAUNA IN WESTERN NEW YORK, PART 2 297 



and closely incurved, beaks acute, small, and directed forward. Hinge line 

 straight, shorter than the length of the shell, making nearly a right angle 

 with the periphery at its anterior extremity, but a much larger angle at the 

 posterior extremity. 



As specific characters in this genus rest wholly on permanent variations 

 in the external ornament, this feature is to be depicted with care, and 

 herein our observations are not based alone on internal and sculpture casts, 

 but have been materially aided by barite replacements in which the contour 

 of the detail is retained with absolute accuracy. 



The number of plications in mature specimens is from n to 13. In 

 young shells there may be not more than six or eight, but this number is 

 invariably the accompaniment of diminutive size. In the umbonal regions 

 of full grown shells these plications are distinctly convex, and their lateral 

 slopes pass without interruption into the smooth, concave and narrow inter- 

 vening furrows. Over the median part of the body of the valve the ribs 

 become broader and flattened above, and distinct lateral ridges are gradu- 

 ally developed, which separate each rib from the sulcus. These ridges 

 become more elevated toward the ventral margin, where they are raised 

 into low carinae, while the ribs of which they form the boundaries become 

 slightly concave because of their elevation. It is important to keep before 

 the mind this variation with growth in view of the fact that Beushausen 

 has laid much importance on the form of the cross section of the plications 

 as a specific value, without taking into proper account this certainty of vari- 

 ation from early to later stages. 



The surfaces of the ribs between the carinae are marked by quite 

 prominent, retrally curved ridges having their longest slope toward the ven- 

 tral margin. These may sometimes, if rarely, present a slight subangula- 

 tion at the middle, seen best on the extremital ribs. The cross ridges are 

 generally subequidistant, but toward the periphery become crowded and 

 finer. Compression in the shale and complication with the matrix often 

 have the result to obscure these cross markings or to make them appear 

 less regular than they actually are. On the extremital slopes they are 

 somewhat finer than elsewhere. 



