LAMELLIBRA NCHIATA. 605 
Ctenodonta recurva.] 
Crenoponra RECURVA Ulrich. 
PLATE XLII, FIGS. 98—101. 
Tellinomya recurva Uuricn, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 221. 
Shell small or of medium size, compressed-convex, slightly oblique, subtriangu- 
lar, the length and hight almost equal, the thickness equalling about one-third of 
the hight. Rostral portion strongly recurved, ends narrowly rounded, base nearly 
semi-elliptical, with more curvature in the posterior half than in the anterior. 
Beaks prominent, posterior to the center of the shell; umbones with an unusually 
small degree of convexity. Cardinal margins sharply inflected, forming an elongate 
depressed area on the anterior side and a shorter, narrowly cordiform one behind or 
rather beneath the beaks. A shallow and gradually widening sulcus extends from 
the beak along the antero-cardinal margin to the antero-ventral border. Surface 
marked by several strong lines of growth and between them fine concentric strie, 
about ten in 3mm. Hinge plate strong, bent at a right angle, the posterior part 
nearly straight (gently concave), somewhat shorter than the anterior, with about 
twenty small, curved transverse teeth, decreasing, as usual, gradually in size and 
curvature toward the beak; anterior part convex, with about thirty teeth. Consid- 
ering the strength of the hinge plate, the teeth are very small. Immediately in 
front of the beak, in the angle of the hinge, a narrow oblique space breaks the 
continuity of the series of denticles. Hinge plate margined on the outer side by a 
delicate sharp ridge; just within it a narrow furrow which has considerable width 
and depth for some distance in front of the beak. In front of the beak and above 
the marginal line of the hinge plate a small area is defined apparently for the recep- 
tion of an external ligament. (See note, p. 578.) Anterior and posterior muscular 
scars distinct, though not very strongly impressed; as usual for this section of the 
genus in size and form. 
The compressed form reminds of C. compressa, but the shape is different, the 
length being greater, the ends more narrowly rounded and the rostral part more 
strongly recurved. The surface markings also are coarser and the posterior lunettes 
much better defined, while a number of important differences may be observed in 
their hinges. Hall’s C. alta is similar in the basal part, but is a more convex shell 
and much less curved in the rostral part. 
Formation and locality.—Upper beds of the Hudson River group, at several localities in Fillmore 
county, Minnesota. It is associated with the next species (C. similis), but is not nearly so abundant. 
The species occurs, though so far as observed only in the condition of casts, also at Oxford, Waynesville 
and other localities in Ohio, and at Richmond, Indiana. 
