598 THE PALEONTOLOGY OF MINNESOTA. 
[Ctenodonta albertina. 
CTENODONTA ALBERTINA, 7%. Sp. 
PLATE XLII, FIGS. 76—80. 
Shell subovate, widest in the anterior half, 10 to 18 mm. in length; beaks mod- 
erate in size and prominence, situated about 4.5 mm. behind the anterior extremity 
in a specimen 16 mm. long; antero-cardinal region compressed, slightly alated, 
subangular in outline; anterior margin nearly vertical and rather gently convex 
above the lower part, where it turns somewhat rapidly backward into the broadly 
rounded base; the curvature of the basal outline is often not quite uniform, being, 
in these cases, a little stronger in the anterior than in the posterior half; posterior 
margin somewhat obliquely rounded-subtruncate; as shown in the figures the width 
of the posterior end is somewhat variable; cardinal margin nearly straight; umbonal 
ridge rounded, inconspicuous. Surface almost entirely smooth. 
In casts of the interior the beaks are prominent, compressed and very little 
incurved, the adductor muscular scars are distinct, the posterior one being especially 
prominent and the larger, while the anterior one is drawn out above almost to the 
point of the beak; the posterior cardinal outline is strongly concave, while the dor- 
sum in this part is formed by a sharp curved ridge running backward from each 
beak to the adductor scars and enclosing the area that had been occupied by the 
hinge plates. The hinge plate is strong, contracted and bent beneath the beak, the 
posterior part one-third longer than the slightly declining anterior part; denticles 
strongly geniculated and deeply interlocking, the continuity of the series distinctly 
interrupted under the beaks by an illy defined pit-like space. In five valves the 
total number of denticles ranged from twenty-nine to thirty-two, with thirteen, 
fourteen or fifteen anterior and sixteen or seventeen posterior. In a sixth valve, 
unusually short and possibly not belonging to this species, there are only eleven 
anterior and thirteen posterior teeth. 
Length of an average example, 12 mm.; hight, 9.5 mm.; thickness,6mm. Ina 
large specimen these dimensions are respectively 16, 12.2 and 7 mm. 
The type of structure exhibited in this species and in C. filistriata, C. madisonensis 
and two as yet unpublished forms from the Cincinnati group of Ohio and Kentucky, 
stands somewhat apart from the other two types (7. e. C. nitida and C. fecunda) 
included in this section of the genus. The geniculated hinge teeth and the pit 
beneath the beak are peculiar features, while another difference, when compared 
with the C. nitida type, appears in the absence of the small accessory scars over the 
adductors. That some importance attaches to the absence of these small scars is 
indicated by their constant presence in the species which pertain strictly to the 
nitida type in other respects. These scars seem to be wanting in the shell of the 
