LAMELLIBRANCHIATA. 587 
Ctenodonta cuneiformis.] 
CTENODONTA CUNEIFORMIS, %. Sp. 
PLATE XLII, FIGS. 31—3. 
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 4mm. 
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 strie. 
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 1.5 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 not contracted in the same manner, and the anterior outline more 
uniformly rounded. 
Formation and localityx—Four specimens were found at a point about six miles south of Cannon 
Falls, 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. 
CYrENODONTA GIBBERULA Sa/ter. 
PLATE XLII, FIG. 37 
Ctenodonta gibberula SALTER, 1857. Canadian Organic Remains, Dec. 1, p. 38. 
Tellinomya ventricosa HALL, 1861. Rep. Supt. Geol. Sur. Wis., p. 27; 1862, Final Report of same, 
p. 38, fig. 8; MEEK and WoRTHEN, 1868, Geol. Sur. Ill., 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 
