LAMELLIBRANCHIATA. 601 
OCtenodonta intermedia.] 
Adductor sears subovate, situated immdeiately beneath the ends of the hinge, dis- 
tinct, the posterior one the deeper and margined on the inner side by an obtuse 
ridge-like swelling. Small accessory scars have not been observed. 
A single imperfect valve was all I had seen of this species when I first deseribed 
it. During the summer of 1892, however, I succeeded in collecting an excellent 
series of specimens, so that Iam now enabled to present the shell in all its characters 
and to point out those which are really distinctive. Compared with C. astartiformis 
Salter, of which an authentic example is now before me, it differs externally in its 
greater proportional width, somewhat narrow posterior curve, less convex valves, 
finer concentric lines and in wanting the coarse wrinkles of growth which seem to 
be a constant feature of the ventral half in that species. Internally the muscular 
scars and the denticles of the hinge are about the same in the two species, but the 
hinge plate is considerably wider in the Minnesota form, while the flat space beneath 
the denticles of the latter is scarcely represented in Salter’s species. Casts of the 
interior of the two species are not easily distinguished, the only reliable differences 
between them, so far as observed, being the lesser prominence and more uniform 
curvature of the anterior margin and the slightly greater convexity of the casts of 
C. astartiformis. 
Formation and locality.—Upper part of the middle third of the Trenton shales at several localities 
in Goodhue county, Minnesota. Casts belonging to this species or to C. astartiformis, the latter probably, 
have been found in the upper part of the Trenton limestone at Minneapolis and at Janesville, Wisconsin, 
and I have specimens of a very similar, though smaller, form from the upper third of the Trenton shales. 
CTENODONTA INTERMEDIA Ulrich. 
PLATE XLII, FIGS. 95-97. 
Tellinomya intermedia ULnicn, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 218. 
Shell thin, of medium size, moderately ventricose, rather erect, the hight a little 
greater than the length. Outline subtriangular, at the beaks, which are obtusely 
acuminate and incurved, forming very nearly a right angle; anterior cardinal margin 
very gently convex, posterior cardinal edge correspondingly concave, ventral margin 
together with the curve into the ends forming a semicircle. Ends subequal, the 
posterior sometimes a little the longest. Umbones full, the remainder of the surface 
sloping uniformly to the free margins. An obscure sulcus may be detected near the 
anterior margin, and along the dorsal part of this end the surface descends abruptly 
to the hinge plate. Surface with strong, closely arranged, thread-like, concentric 
lines, about twelve in 5 mm. At intervals of about 2 or 5 mm. generally a fold 
stronger than the rest. 
Casts of the interior exhibit a faint ridge and sulcus in the anterior end, and 
