LAMELLIBRANCHIATA. 589 
Ctenodonta carinata.] 
CTENODONTA CARINATA, 2. Sp. 
PLATE XLII. FIGS. 41—43. 
Shell rather small, gibbous in the anterior and rostral portions, the ends obtuse 
in a dorsal view, the beaks large, prominent, strongly incurved, situated near the 
midlength and turned decidedly toward the posterior end; the outline may be 
described as subtriangular or obscurely quadrate; anterior margin most prominent 
and strongly rounded in the lower half, the upper two-thirds more gently curved 
and sloping backward to the beaks, being continuous with the antero-dorsal margin; 
ventral margin straight or broadly sinuate, curving abruptly into the subtruncate 
posterior margin and forming, with the latter, an angle of about 80°; cardinal out- 
line distinctly concave behind the beaks; post-cardinal region slightly produced, 
though too blunt to be called alate; posterior umbonal ridge prominent, angular, 
extending to the post-ventral angle; point of greatest convexity on the rounded 
anterior umbonal ridge; between the two ridges a wide, undefined sulcus, extending 
from the umbones to the base. Surface marked with distinct and rather irregular 
concentric lines of growth. Hinge plate arcuate, in other respects apparently as in 
C. gibberula Salter. Shell thick, muscular scars not observed. 
C, gibberula is the only shell known to me with which C. carinata might be 
compared. Although imperfectly known, I am quite confident that its affinities lie 
chiefly with that species. Still, though the resemblances are sufficient to prove that 
the two forms belong to the same section of the genus, it is scarcely likely that any 
one will fail to distinguish them specifically, the outline in the two species being 
different in several respects. Thus, in C. carinata the posterior end is wider, the 
post-cardinal region produced and subcuneate instead of flattened, the anterior 
margin is more prominent below, and above curves more regularly into the dorsal 
outline, while the basal margin is not so prominent anteriorly and on the whole 
more nearly horizontal. The posterior umbonal ridge also is more prominent, the 
mesial sulcus or flattening is a more pronounced feature and the anterior slope more 
abrupt. Finally, the hinge plate is less bent and curved rather than geniculated. 
Formation and locality.—Middle Galena, about one mile east of Fountain, Minnesota. 
CrmnopontTa PLANODORSATA Ulrich. 
PLATE XXXVII, FIGS. 25-28; PLATE XLII. FIGS. 38—40, 
Tellinomya planodorsata ULRICH, 1892. Nineteeth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 217. 
Shell small, moderately convex, subtriangular or trapezoidal, the hight, length 
and thicknes, respectively, ten, fourteen and five mm.; beaks small, incurved, scarcely 
projecting above the hinge, situated nearly one-third of the entire length from the 
