500 THE PALEONTOLOGY OF MINNESOTA. 
. Pa 
[Byssonychia tenuistriata . 
ByssoNnYCHIA TENUISTRIATA, #1. Sp. 
Vig. 39. Byssonychia tenwistriata, n. sp. Hudson River group, Granger, Minnesota. The right side 
and a front view of an imperfect cast of the the interior. Mus. Reg. No. 8371. 
Shell rather small, subovate, moderately ventricose in the umbonal region and 
anterior half, compressed in the postero-cardinal region where the surface is dis- 
tinctly concave; anterior slope strongly convex, but scarcely abrupt; beaks small, 
projecting but little, moderately inecurved. Hinge line comparatively short, the 
outline passing rather gently into the broadly-rounded posterior margin; basal line 
strongly convex, curving uniformly into the ends; anterior side slightly concave 
above, neatly convex below. Byssal opening small, its position high, it and the 
surface around it appearing in casts as a distinct impression immediately beneath 
the beaks. Surface marked with very fine radiating strie and obscure concentric 
varices of growtb, both showing through the marginal parts of the shell, so as to be 
visible on good casts of the interior. The total number of the radiating striz is 
probably more than seventy. Near the base of the specimen figured eleven were 
counted in the space of 5 mm. 
This species is closely related to B. vera Ulrich, (see ante p. 479, fig. 36, V) 
from the Utica horizon of the Cincinnati group of Ohio, differing from it chiefly in 
its finer radiating strize and more impressed byssal opening. B. intermedia M. and 
W., of the Galena, has coarser strive and is a more ventricose shell. 
Formation and locality.—Rare in the upper part of the Hudson River rocks at Granger and Spring 
Valley, Minnesota, and in an equivalent position at Richmond, Indiana. 
Mus. Reg. Nos. 8370, 8371. 
Family MODIOLOPSIDAj, n. fam. 
Shell equivalved, usually elongate ovate, but varying to oblong subquadrate, 
generally thin; valves fitting closely or gaping slightly at one or both ends. Beaks 
near the anterior end, but never terminal. Hinge long, of variable strength, edentu- 
lous or with one or two cardinal teeth in one or both valves. Ligament long, linear, 
external and internal. Anterior adductor impressions rather large and distinct, 
situated between the beaks and the anterior extremity; above them a very small 
