562 THE PALEONTOLOGY OF MINNESOTA. 
[Vanuxemia decipiens. 
postero-basal part a little more curved than elsewhere. Surface rather uniformly 
convex in the central and umbonal regions, with the point of greatest convexity a 
trifle in front of the middle and unusually low. 
Casts of the interior show that the shell was thickened internally on the anterior 
part, that the posterior side of the thickening was margined by a slightly oblique 
narrow groove or sulcus which left a blunt though well marked ridge on the cast 
extending down from a little behind the beaks to below the middle of its sides. 
Beaks very prominent in the casts, greatly compressed, but little incurved, hollow 
upon the inner side, an unusually large space left between them, showing that the 
hinge plate was much thickened in this part. The exact width of the hinge plate is 
unknown, but it must have been considerable and probably greater than the average, 
especially at the ends where it was bent down to make room for the large teeth. 
Of the cardinal teeth there were three in the right valve and the same number or 
only two in the left. They were of large size and rather strongly curved and oblique. 
The posterior teeth were strong, scarcely curved and oblique, but their number is 
unknown. Anterior muscular impression of medium size, rounded, sharply defined, 
but not very deep, excavated out of the anterior end of the hinge plate, the pair — 
forming (in an anterior view of the cast) a narrow lobe partly between, but mostly 
in front of the filling of the beaks. Pallial line sharply defined in the anterior half, 
obscure behind. Posterior muscular impression very faint, large, situated immedi- 
ately beneath the lateral teeth. A large cast has a hight of over 50 mm. 
This shell is the largest known to belong to the genus. It belongs to the group 
of species of which V. hayniana Safford may be regarded as typical, but differs from 
them all in having the point of greatest convexity situated at the center instead of 
above the center. It is scarcely necessary to compare the species in detail with the 
numerous forms to which it is more or less nearly related, since ordinary specimens 
are distinguished at once by their unusual size. 
Formation and locality.—Middle or upper part of the Galena, Mount Carroll, Illinois. 
VANUXEMIA DECIPIENS, ”. Sp. 
PLATE XXXIX, FIGS. 1-5. 
Shell rather small, strongly convex, obliquely ovate, highest posteriorly, the 
length of a large specimen 23 mm., its hight 18 mm. Hinge line straight, two- 
thirds as long as the shell, terminating subangularly behind, rounded in front; 
posterior margin slightly oblique, strongly rounded and somewhat prominent in the 
lower half; basal line moderately convex, ascending from the posterior third or 
fourth; anterior end narrowly rounded, very short, the greater part of it occupied 
