LAMELLIBRANCHIATA. 605 
Ctenodonta hamburgensis.] 
the outline may be called subcircular, the basal half, as a rule, being quite regularly 
curved; length and hight nearly equal, the latter dimension commonly a little the 
greater. Beaks prominent, situated behind the center, turned backwards. Surface 
marked by comparatively strong concentric lines. 
On casts of the interior, and this is almost invariably the condition in which the 
species is preserved, the muscular scars are nearly always distinguishable and the 
posterior one is often sharply defined and prominent on the upper side. They are 
situated just within the ends of the shell and each near the wider and lower end of 
an obscurely defined sulcus. The two sulci, of which the anterior one is usually the 
better marked, begin near the beak and extend down on each side to the base of the 
muscular scars. A small accessory scar has been observed immediately above the 
posterior adductor. Pallial line simple, rather distinct. Hinge plate comparatively 
strong, with numerous (at least thirty) small denticulations. 
The size of this shell varies greatly. Many of the specimens found at Cincinnati 
- and localities in the vicinity of that city are less than 2 mm. in diameter, but others 
are occasionally met with that range from that size to a diameter of 5mm. In the 
northwestern localities the species grew to a larger size, specimens having a diameter 
of from 5 to 7 mm. being in the majority. Aside from the matter of size, however, 
the specimens from these two regions are practically identical. 
Dr. S. A. Miller, in the work above referred to, erects a new genus, Palcoconcha, 
and a new family for the reception of the present species, which he divides into two 
species, giving to the larger form the specific name faberi. But this new genus and 
family have no right to recognition, since they are based entirely upon erroneous 
observation, he having come to the conclusion that the hinge in these shells was 
not denticulated and probably edentulous. Through the kindness of Dr. Miller I 
had an opportunity to examine a number of the specimens (excellent casts of the 
interior) used by him in defining his genus. Even among these I noticed several 
that retained undeniable evidence of the denticulate hinge. 
Formation and locality.—Very abundant at Cincinnati, Ohio, and numerous other localities in the 
the vicinity of that city. In the northwest it is one of the rare fossils of the so-called ‘‘Nucula bed” 
of the Maquoketa or Hudson River shales. 
CTENODONTA HAMBURGENSIS Walcott. 
PLATE XLII, FIGS. 91 and 92. 
Tellinomya? Hamburgensis WALCOTT, 1884. Pal. Eureka District, p. 76. 
Shell small, rather convex, rounded-subrhomboidal in outline, with the hight 
and length subequal and the beak comparatively large, incurved and situated in 
front of the center; posterior dorsal margin somewhat straightened. Surface 
