616 THE PALEONTOLOGY OF MINNESOTA. 



[Technophorus dlvarlcatus. 



widening shallow sulcus and the straightening of the ventral margin. Postero- 

 cardinal slope concave, narrow, descending rather rapidly, not well preserved in the 

 specimen. Interior unlmown; shell substance very thin. 



Length 21 mm., hight 12.5 mm., greatest convexity (of a left valve) 2.5 mm. 



Formation and locality.—Vp^ei part of the middle third of the Trenton shales near Cannon Falls, 

 Minnesota. It is associated with Plethocardia unibonata, Ctenodonta planodorsata, Matheria rugosa and 

 other shells characterizing this horizon. 



Technophorus divaeicatus TJlrich. 



PLATE XJL, FIGS. 37 and 38. 



Technophorus divarieatus Ulbich, 1892. Amer. Geol. vol. x, p. 102. 



Shell small, moderately convex, elongate, the length a little more than twice the 

 hight. Beaks small, scarcely projecting above the hinge line, situated about one- 

 third of the entire length from the anterior extremity. Dorsal margin nearly 

 straight, (faintly concave on each side of the beaks) about three-fourths as long as 

 the shell, terminating abruptly where it joins the concave posterior edge, with the 

 upper part of which it forms an angle little short of 90°. Anterior end a little higher 

 than the posterior, strongly rounded in outline, especially above; below rounding 

 neatly into the at first gently convex, then straight and finally concave basal line. 

 Posterior ridge thin but very prominent, curving slightly in its course from the beak 

 to the sharply produced postero-basal angle. Surface uniformly convex and marked 

 with fine, thread-like concentric lines in the antero-basal three-fifths beyond which 

 it first descends into a sulcus and then ascends sharply to the summit of the ridge, 

 dropping on the other side even more abruptly into the wing-like postero-dorsal part 

 of the shell. On each side of the posterior ridge there are distinct divaricating lines, 

 twice as strong as the concentric lines on the anterior part of the shell. They join each 

 other on the ridge, while those on the lower side of the latter meet the concentric 

 lines at angles of about 70°. Finally there is another set of such lines along the dorsal 

 edge, running parallel with the set on the lower side of the ridge. Under a magni- 

 fier, with certain lights, these lines appear as though minutely crenulated. Internal 

 characters unknown; shell substance very thin. 



Length 12.5 mm., hight at the beaks, 5.8 mm., hight at posterior end of hinge, 

 5.1 mm., greatest thickness of closed valves, 4.1 mm. 



Casts of the interior would be distinguished by having the dorsal and ventral 

 margins more nearly parallel than is the case in any of the other species referred to 

 the genus, except T. punctostriatus of the Cincinnati group, which is, however, a 

 shorter shell, and widely different in other respects. With the shell in a good state 



