^, ^ LAMELLIBBANCHIATA. 595 



Ctenoaonta 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 G. alhertina, in which the larger side is undeniably 

 the posterior. 



Formation and locality.— TMs small shell occurs in great numbers in certain layers of the middle 

 third of the Trenton shales at St. Paul, Minneapolis, Cannon Falls, Chatfleld and other localities in the 

 state. The surface of a layer may he completely covered by separated valves or by casts of the interior. 

 The la,tter condition is the prevailing one at the tv?o 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. Beg. No. 8627. 



Ctenodonta fecunda Hall. 



PLATE XLII, FIGS. 67—73. 



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 umbdnes 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 

 striae 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. As a 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. scq- 

 fieldi, the atiterior end being narrower and in two cases also shorter, while th^ 



