^ ^ LAMELtlBRANOHIAT^A. 603 



Ctenodonta l^eourVa.l 



Ctenodonta eecurva Ulrich. 



PLATE XMI, FIGS. 98-101. 



Tellinomya recurva Uleich, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 221. 



Shell small or of medium size, compressed-convex, slightly obliqiie, 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 strise, 

 about ten in 3 mm. 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 0. alta is similar in the basal part, but is a more corfvex shell 

 and much less curved in the rostral part. 



Formation and Zocaiiij/.-^Upper beds of the Hudson River group, at several localities in Fillmore 

 county, Minnesota. It is associated with the next, species (O. similis), but is not nearly so abundant. 

 The species occurs, though so far as observed only in the cdndition ofcasts, also at Oxford, Waynesville 

 and other localities in Ohio, and at Richraond, Indiana. 



