LAMELLIBRANCHIATA. 591 
Ctenodonta logant.] 
convex, straight behind the beaks; anterior end short, semicircular; ventral margin 
gently convex; posterior end a little narrower than the anterior and more sharply 
rounded. Surface with obscure concentric lines; sloping rapidly at the cardinal 
margin, but very gently to the ends and ventral edge. Hinge plate of moderate 
strength, bent a little beneath the beak and with a thickening on the lower side in 
front of same. Posterior to the beak the plate is long, straight and bears twenty or 
more small teeth, while on the anterior part only nine are to be counted. In the 
vicinity of the beak the teeth, especially those on the posterior side, are very small, 
and as they are all set at right angles to the hinge plate, the continuity of the series 
is interrupted where the two series come together. The interruption is easily over- 
looked, because of the slight bend in the hinge plate. Anterior muscular impression 
deep, situated immediately beneath the end of the hinge. Its posterior side is defined 
by a strong vertical thickening of the shell, in the upper part of which the scar of a 
small pedal muscle is to be observed. Posterior scar distinct, but less sharply 
impressed than the anterior, situated at the end of the hinge just within the thin 
post-cardinal border of the shell. 
_ The characters of the hinge and the deep muscular impressions show that this 
species is related to C. planodorsata, with which it is also associated in the shales. 
There is, however, room for several intermediate species, the form being much more 
elongate, the back not flattened, the posterior end rounded instead of subacute, and 
the hinge much less bent. In C. subnasuta the shape is somewhat similar, but the 
hinge is different, the muscular impression not nearly so distinct, the beaks larger 
and situated farther from the anterior end, while the anterior half is relatively 
higher. 
Formation and locality.—Middle third of the Trenton shales, Goodhue county, Minnesota; asso- 
ciated with C. planadorsata, C. compressa, C. socialis and C. scofieldi. 
C. logani section. 
CTENODONTA LOGANI Salter. 
PLATE XLII. FIGS. 26—28. 
Tellinomya dubia HALM, 1857. Tenth Ann. Rep. Reg. Univ. N. Y., p. 183, figs. 4 and 5. (Not 
T. dubia Hall. 1847, Pal. N. Y., vol. 1, p. 153.) 
Ctenodonta logani SALTER, 1859. Canadian Organic Remains, Dec. i, p. 26. (Not C. logani Salter, 
1851, Rep. Brit. Assoc., p. 63, which proved to be the same as 
Tellinomya nasuta Hall, 1847.) 
Shell of the medium size, rather elongate, subovate, strongly convex, very gib- 
bous in the umbonal region, with the strongly incurved beaks turned slightly forward 
and situated near the midlength; posterior end a little the narrower, rounded, but 
not quite uniformly, the lower part being usually a trifle prominent; basal margin 
gently arcuate, the posterior half somewhat less convex than the anterior; anterior 
