NAPLES FAUNA IN WESTERN NEW YORK, PART 2 2 7 1 



development of these ribs in different individuals ; in some they are obscure 

 except at the beak, in others clearly defined over the entire surface. The 

 posterior rib is much broader than the rest and takes in the umbonal ridge. 

 Besides these surface characters there are the following : very fine radial 

 lines of subequal size cover the entire surface, and these are crossed by 

 equally fine concentric lines. The latter make a broad upward turn along 

 the postventral edge where passing over the umbonal ridge. On the 

 cardinal margin there is a short subtriangular area directly beneath the 

 beak, which is not clearly set off from a semicircular emargination (lunule ?) 

 in front. This emargination extends halfway from the beak to anterior 

 extremity, and beyond its outer end the hinge line is straight. No ear is 

 present. Posteriorly the subcardinal area passes into a broader concave 

 depression, which is crossed by two oblique ridges, the surface between 

 being depressed. No evidence is present of ligamental pit or striation, and 

 there is no posterior wing. 



No specimens of this species retain the valves in conjunction ; but, 

 from analogy with the German specimens, we feel secure in referring to 

 it detached right valves having the following characters. Subelliptic, 

 depressed, gently convex over the anterior portion, depressed postmedially, 

 narrowly and often acutely elevated posteriorly. Beak one third the width 

 of the shell from the anterior margin ; umbo projecting beyond the cardinal 

 line, but depressed. The postmedian depression or sinus is oblique, 

 usually very broad, sometimes more sharply impressed, setting off the 

 posterior expansion with the aspect of a wing. The latter, on account of 

 its marginal elevation, must have bounded a wide posterior siphonal open- 

 ing, which had a more extensive cover on this than on the left valve. No 

 trace of hinge structures. Surface with obscure traces of broad radial ribs 

 on the convex body and finer radial lines specially marked on the posterior 

 parts. Fine concentric lines are also present. 



The shell substance of the species is very thin, and none of the 

 specimens bear indication of musculature. 



Observations. This species is perhaps the commonest of the represen- 



