LAMELLIBRANCHIATA. 499 
Byssonychia intermedia.] 
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 Cincinnati 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 WORTHEN, 1868. Geol. Sur. Ill., 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. Beaks 
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. They 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 
cost are not defined except at the free margins, the rest of the surface being smooth. 
Muscular 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, pl. 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.—Galena limestone, Mount Carroll, Illinois; Oshkosh, Wisconsin, and near 
Wykoff, Minnesota. 
Mus. Reg. No. 8359, 
