562 THE PALEONTOLOGY OF MINNESOTA. [Vanuxemla declplens. 



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 niiddle of its sides. 

 Beaks very prominent in the casts, greatly compressed, but little incurved, hollow 

 upon the inner, side,, an unusaally 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 harrow 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.— MiA&iQ or upper part of the Galena, Mount Carroll, Illinois. 



Vanuxemia deoipiens, n. 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" 



