LOWER SILURIAN LAMELLIBRANCHS. 



639 



make an estimate tfyit will not be far from the truth. My judgment 

 places the number at about twenty on each valve. Comparing these 

 with about thirty for A. alata and not less than thirty-six in A. gigantea, 

 the result is, especially in shells of the same size, a much more coarsely 

 plicated surface for A. plicata. 



Formation and locality: Middle beds of the Cincinnati group, Cin- 

 cinnati, Ohio. 



Genus ERIDONYCHIA, n. gen. 



Like Byssonychia and Anomalodonta, excepting that the hinge is edentu- 

 lous. Type: E. apicalis, n. sp. 



, This genus includes, as far as known, a small and comparatively un- 

 important group of Lower Silurian shells, agreeing with Byssonychia in 

 all respects except that their hinges are entirely without cardinal and 

 posterior lateral teeth. A well defined, striated ligamental area however 

 is present, and in the type species several obscure and irregular small 

 ridges beneath the posterior extremity of the external area, remind of 

 the internal ligament supports of Anomalodonta. But the oblique cardi- 

 nal fold of the latter genus is not represented, and the acuminate beaks 

 and oblique form of the shells gives them a peculiar expression, so that 

 no other course seemed open than to erect a distinct group for their 

 especial benefit. If we could decide with which of the two genera, Bys- 

 sonychia or Anomalodonta, the affinities were greatest, it might be ad- 

 visable to reduce Eridonychia to the rank of a subgenus. 



Eridonychia apicalis, n. sp. 



Plate 47, Fig. 1. 



Shell oblique, ovoid, with prominent, subacute and very little in- 

 curved beaks; hinge short, edentulous; ligamental area erect, wide 

 beneath the beaks, tapering posteriorly, marked with four or five hori- 

 zontal grooves; several obscure folds within the posterior extremity of 

 the hinge; byssal opening long, narrow, its border sharply inflected; at 

 its lower end the outline of the valves is slightly produced and rather 

 abruptly rounded. Surface with about thirty distinct rays, separated by 

 flattened interspaces as wide as the costae. 



This species resembles Byssonychia acutirostris very closely, and col- 

 lectors will no doubt find it difficult to separate them when the specimens 

 are not very good. A careful comparison shows that the margin of the 

 byssal opening is more sharply inflected and the beaks, especially in casts, 

 more erect in the Eridonychia. The principal differences however lie in 

 the hinge, that species having true cardinal teeth and a narrower liga- 



