LAMELLIBRANCHIATA. 587 



Otenodonta ouneiformls.] 



Ctenoijonta cuneiformis, n. sp. 



PLATE XLTI, FIGS. 31-33. 



Shell small, compressed convex, transversely somewhat acuminate-ovate, tapering 

 posteriorly to a narrowly rounded extremity; anterior end shorter than the posterior, 

 but much higher and broadly rounded, except in the antero-cardinal region, where 

 the outline projects slightly beyond the path of a uniform curve; base rather prom- 

 inently rounded in the middle, convex throughout the anterior half, straight or very 

 faintly sinuate in the posterior half', beaks small, not prominent, situated about 4 mm. 

 behind the anterior extremity in a specimen 11 mm. long; posteriorly from the beaks 

 the cardinal outline is straight, in front of them gently concave. Behind the center 

 the shell is more or less distinctly contracted. Surface with obscure concentric striae. 

 Hinge plate of moderate strength, comparatively long, very gently bent, and just 

 appreciably contracted beneath the beaks, with about twenty-seven nearly vertical 

 teeth in each valve, twelve in front of the beak. The posterior six or seven teeth are 

 stronger than the rest and bent inward. Test thin, muscular scars not observed. The 

 largest specimen seen, a right valve, is 12 mm. long, 7.4 mm. high and 1.8 mm. thick. 



The contraction and narrowness of the posterior end gives to this species some- 

 what the appearance of C. contracta Salter, but the two species are really widely 

 different.' The Canadian shell is higher, more convex and its cardinal outline much 

 more angular, the hinge plate strongly bent and very narrow under the beaks, the 

 teeth larger and not so numerous, and the shell much thicker. The affinities of 

 C. cuneiformis are probably with C. nasuta and C. subnasuta. The former, being a 

 much larger shell, is not likely to be confounded with it. The latter is narrower 

 anteriorly and wider posteriorly, is more convex, especially in a front view, its 

 posterior half is nqli contracted in the same manner, and the anterior outline more 

 uniformly rounded. 



Formation and locality. — Four specimens were found at a point about six miles south of Cannon 

 Palls, Minnesota, where they occurred in the upper part of the middle third of the Trenton shales. The 

 same locality and bed has furnished numerous other Lamellibranchiata. 



C. gibberula section. 



Ctbnodonta gibberula Salter. 



PLATE XLII, FIG. 37. 



Ctenodonta gibberula Salter, 1857.- Canadian Organic Eemains, Dec. i, p. 38. 

 Tellinomya ventricosa Hall, 1861. Rep. Supt. Geol. Sur. Wis., p. 27; 1862, Final Report of same, 



p. 38, flg. 3; Meek and Wobthbn, 1868, Geol. Sur. 111., vol. iii, p. 307. 



Shell rhombic subovate, ventricose, the hight, length and thickness, respectively, 

 as seven, ten and six, with large incurved beaks, situated a little behind the mid- 

 length; antero-dorsal and ventral margins subparallel, the posterior end obliquely 



