616 THE PALEONTOLOGY OF MINNESOTA. 
(Technophorus divaricatus. 
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 unknown; shell substance very thin. 
Length 21 mm., hight 12.5 mm., greatest convexity (of a left valve) 2.5 mm. 
Formation and locality —Upper part of the middle third of the Trenton shales near Cannon Falls, 
Minnesota. It is associated with Plethocardia umbonata, Ctenodonta planodorsata, Matheria rugosa and 
other shells characterizing this horizon. 
TroHnopHorus pivaRricatus Ulrich. 
PLATE XL, FIGS. 37 and 38. 
Technophorus divaricatus ULRICH, 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 7. punctostriatus of the Cincinnati group, which is, however, a 
shorter shell, and widely different in other-respects. With the shell in a good state 
