LAMBLLIBRANCHIA.TA. 499 



Byssonychia Intermediii.] 



The Ambonychia intermedia Meek and Worthen, of the Galena, seems to be the 

 earliest species of Byssonychia now known. Perhaps contemporaneous with this is a 

 form, occurring in the Trenton of Kentucky and Tennessee, that is scarcely distin- 

 guishable from the Hudson River B. radiata. Nine or ten additional species, of 

 which two only are described {A. retrorsa and robusta, of Miller) occur in the Hudson 

 River and Cihcinnati rocks. So far as known the genus became extinct. with the 

 close of the Lower Silurian. , 



Byssonychia intermedia Meek and Worthen. 



PLATE XXXV, FIGS. 23—26. 



Ambonychia intermedia Meek and Wokthbn, 1868. Geol. Sur. 111., vol. iii, p. 306. 



Shell small, rhombic-subovate, the length and hight about as eleven is to fourteen; 

 gibbous in the umbonal, anterior and central regions, compressed and subalate postero- 

 dorsally. Hinge line a little shorter than the greatest antero-posterior diameter of 

 the valves, ranging at an angle of about 90° with the anterior margin. Anterior side 

 truncated nearly vertically above, below rounding backward into the base, the outline 

 around the lower two-fifths of the shell forming nearly a regular semicircle. Poste- 

 rior margin straightened above or rounding regularly into the hinge line. Beg^ks 

 prominent, full, obtusely pointed, strongly incurved with a slight forward direction. 

 Internal casts are somwhat excavated in the upper part of the front in the space 

 surrounding the small byssal opening, and between the latter and the points of the 

 beaks there is a small protuberance representing the filling of a little cavity at the 

 extremity of the hinge plate. Surface marked by rather fine radiating plications, 

 the total number, as near as can be determined from casts, being between forty-five 

 and fifty. Tliey are coarser on the ventral slope than on the posterior wing, always 

 simple and increase in strength with the growth of the shell. On large casts the 

 costse are not defined except at the free margins, the rest of the surface being smooth. 



Muscftlar scar and pallial line unusually obscure, their positions and .form not 

 certainly determined. 



This little shell is a true Byssonychia and quite different from Ambonychia belli- 

 striata Hall, with which Meek and Worthen compare it. It is related to the follow- 

 ing species, but a nearer ally is found in the B. vera Ulrich, of the lower part of the 

 Cincinnati exposures. That species, of which an excellent internal cast is figured on 

 page 479 (fig, 36, pi. V), is less gibbous, more oblique and has smaller beaks, while 

 the muscular scars and pallial line are usually more distinctly impressed. 



Formation and locality.— Ga,lQDa. limestone, Mount Carroll, Illinois; Oshkosh, Wisconsin, and near 

 Wykoff, Minqesota. 



Mus. Reg. No. 8359. 



