LOWER SILURIAN LAMELLIBRANCHS. 



629 



Class LAMEIyLIBRANCHIATA. 

 Family AMBONYCHIIM), Miller. 



Genus Byssonychia, Ulrich, 1894. 

 ( Paleontology of Minnesota, Final report vol. Ill, p. 498.) 

 Byssonychia vera n. sp. 

 Ambon chiabelhstriata, S. A. Miller, 1874, Cin. Quart. Jour. Sci., vol.1, p. 14. 



(No 



Figs. a. and b, the left s-ide and an anterior view of a partial caj-t of the exterior; c, ex- 

 cellently preserved, cast of the interior, showing the pallial and muscular impressions, Utica 

 horizon ot the Cincinnati group, Newport, Ky. 



This name is proposed for the form which, for more than twenty 

 years, has been erroneously identified by Cincinnati collectors with Hall's 

 Ambonychia bellistriata. That species is restricted to the Trenton and 

 differs from B. vera in its greater obliquity, longer hinge line and finer 

 strise, and more importantly in wanting a byssal opening as well as post- 

 erior lateral teeth. 



From B. radiata Hall, sp., the type of the new genus, B. vera differs 

 in its smaller size, finer strise, (there being about fifty to irom thirty- 

 seven to forty in the typical form of that species,) shorter hinge line, more 

 evenly convex valves, and shorter byssal opening. A new variety of 

 that species, found near the tops of the hills at Cincinnati, and which 

 may be called B. radiata var. approximata, approaches B. vera in being as 

 a rule of smaller size than the typical form and in having the number of 

 the radii increased to as high as forty-five* In all other respects 

 however this variety is the same as the usual form of B. radiata. 



B. vera, I am satisfied, is not a descendant of the Trenton variety of 

 B. radiata, but of the Galena B. intermedia, Meek and Worthen, sp. That 

 species is the earliest known and B. vera the second, of a group of spe- 

 cies in which the hinge is short, the beaks and the anterior part of the 

 valves tumid, and the byssal opening short and thickened on the inner 

 margins so as to leave an unusually deep and abrupt depression beneath 



• A comparison of hundreds of specimens of the various species of Byssonychia 

 has shown that wit: in reasonable limits the number of the radiating costae is con- 

 stant for each species, and the same in specimens of all ages. Young specimens 

 therefore appear to have been much more finely striated than the old shells, but a 

 count will show, as also will a comparison of the young shell with the rostral por- 

 tion of the old, that the number of the strise is approximately the same in both. 



