LAMELLIBRANCHIATA. 595 
Ctenodonta fecunda.] 
posterior position of the beaks are features that render the identification of this 
species unusually easy. I hesitated to say whether it should be regarded as nearer 
C. nitida or those ovate shells, like C. albertina, in which the larger side is undeniably 
the posterior. 
Formation and locality—This small shell occurs in great numbers in certain layers of the middle 
‘third of the Trenton shales at St. Paul, Minneapolis, Cannon Falls, Chatfield and other localities in the 
state. The surface of a layer may be completely covered by separated valves or by casts of the interior. 
The latter condition is the prevailing one at the two localities first mentioned, but in Goodhue and 
Fillmore counties testiferous examples are the rule. In central Kentucky the species is occasionally 
met with in the Modiolodon oviformis beds of the Trenton. 
Mus. Reg. No. 8627. 
Crenoponta FEcUNDA Hall. 
PLATE XLII, FIGS. 677. 
Nucula ( Tellinomya) fecunda HALL, 1862. Geol. Sur. Wis., vol. 1, p. 55. (Figured, but not described.) 
Shell small, 9 mm. to 13 mm. in length, rather ventricose, transversely ovate or 
obscurely subrhomboidal in outline, with the umbones rather prominent and full, 
and the beaks incurved, directed slightly forward and situated about one-third of 
the length behind the anterior extremity; base usually a little prominent in the 
middle, somewhat straightened, or at any rate less convex in the posterior than in 
the anterior half; posterior end narrower than the anterior, the outline sloping 
forward rapidly above the produced lower part and merging almost gradually into 
the post-cardinal margin; antero-cardinal outline more or less distinctly concave; 
posterior umbonal ridge rounded. Surface marked by very fine, regular concentric 
strie and strong wrinkles of growth, crossed by delicate radial lines, the network 
thus formed requiring a magnifying lens to make it plainly visible. The radial 
lines, however, are not often preserved. : 
The majority of the specimens seen are casts of the interior, mostly in an excel- 
lent state of preservation. Asa rule, they are marked by a limited number of obscure 
concentric furrows. The muscular scars and pallial line are always faintly defined 
Hinge plate rather narrow, arcuate, nearly two-thirds as long as the shell, with about 
eighteen denticles in each valve; denticles very small under the beaks, where the 
series seems also to have been interrupted by a small space; on each side of the 
beaks they become larger gradually and at the same time assume an oblique direc- 
tion, the upper ends of the teeth being turned away from the beaks. 
Three specimens, illustrating slight variations, have the following dimensions: 
Length, 10,5, 11.0 and 13.0 mm.; hight, 7.0, 8.0 and 10.0 mm.; thickness, 4.5, 6.5 and 
6.8 mm. 
This very common shell is certainly distinct from C. levata, C. nitida and C. sco- 
fieldi, the anterior end being narrower and in two cases also shorter, while the 
