500 THE PALEONTOLOGY OF MINNESOTA. 



tByssonycbui teuuisiriaUi . 



BYSSONYCHIA TENUISTRIATA, n. sp. 



Fig. 39. Byssonychia tenuistriuta, n. sp. Hudson Biver group, Granger, Minnesota. The. right side 

 and a front view of an imperfect cast of the the interior. Mus. Keg. No. 8371. 



Shell rather small, subovate, moderately ventricose in the urabonal 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 incurved. 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 striae and obscure concentric 

 varices of growth, 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 stria- 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. 471), fig. 36, V) 

 from the Utica horizon of the Cincinnati group of Ohio, differing from it chiefly in 

 its finer radiating stria; and more impressed byssal opening. B. intermedia M. and 

 W., of the Galena, has coarser striae and is a more ventricose shell. 



Formation and locality. Ilare in the upper part of the Hudson Kiver rocks at Granger and Spring 

 Valley, Minnesota, and in an equivalent position at Richmond, Indiana. 



Mus. Reg. Nos. 8370, 8371. 



Family MODIOLOPSID^], 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 



