^, , LAMELLIBEANCHIATA. 591 



Ctenodonta loganl.] 



convex, straiight 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, iri the upper part of which the scar of a 

 small pedal muscle is to be observed. Posterior scar distinct, bu fc less sharply 

 impressed than the anterior, situated at the end of the hinge just within the thin 

 ^ost-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 O. planadorsata, C, compressa, C. soeialis and C. scofieldi. 



C. logani section. 

 Ctenodonta logani Salter. 



PLATE XLII. FIGS. 26—28. 



Tellinomya dubia Hall, 1857. Tenth Ann. Rep. Reg. Univ. N. Y., p. 183, figs 4 and 5. (Not 



T. dubia Hall. 1847, Pal. IS. Y., vol. 1, p. 153.) 



Ctenodonta logani Salter, 1859. Canadian Organic Remains, Dec. i, p. 36. (Not O. 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 strbngly incurved beaks turned slightly forward 

 and situated near the midlength; posterior end a little the narrower, rounded, but 

 not quite uniformly, the lower ptirt being usually a trifle prominent; basal margin 

 gently arcuate, the posterior half somewhat less convex than the anterior; anterior 



