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LAMELLIBRANCHIATA 
?Megalodontidwz.] 
umbones less tumid and not so prominent. The impressions of the internal liga- 
ment supports also are very much less distinct. W. subcarinata is not so oblique, 
shorter and has a longer hinge and narrower escutcheon. 
Formation and locality—Galena shales near Cannon Falls, Minnesota. 
Family 7? MEGALODONTID A, Zittel. 
Genus PLETHOCARDIA, Ulrich. 
Plethocardia, ULricu, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 243. 
Shell thin, inequilateral, oblique, tumid, with margins closed; beaks large, 
prominent, spirally enrolled and curving forward. [Posterior cardinal margin with 
a narrow escutcheon or lunette. A strong and large process projects forward and 
downward from the underside of the hinge just beneath the beak in each valve; one 
strong linear lateral tooth, or thickened internal cartilage support, beneath the 
posterior extremity of the hinge line and close to the margin. Anterior muscular 
scar strongly impressed, situated in the antero-dorsal angle, margined on the inner 
side by a curved ridge extending from the under side of the cardinal process. In 
_ casts of the interior the filling of the anterior impressions forms a small but sharply 
defined lobe. Posterior muscular scars indistinct, much larger than the anterior, 
situated just behind the center of the postero-cardinal slope. Pallial line simple, 
submarginal, faintly impressed. 
Type: P.umbonata Ulrich. : 
In the original description of this genus and of the typical species, I called the 
subrostral process a cardinal tooth. This view I now believe to be at variance with 
the facts, for the reason that the supposed tooth does not project beyond the plane 
of the margins of the valve and therefore could not have interlocked with a corres- 
ponding tooth or teeth in the opposite valve. In the left valve, upon which the 
genus and P. umbonata was established, this process was somewhat injured in clear- 
ing away the adhering matrix. It is, however, sufficiently preserved to show that 
it had one large transverse depression in the lower part (for which reason it was 
described as bifid) and probably one or two in the upper part. In an imperfect right 
valve, recently obtained from Kentucky, the process is similarly marked with a large 
depression in the lower part and two (perhaps three) smaller prominences above. 
In neither specimen are the upper prominences in a sufficiently good state of pres- 
ervation to admit of positive declarations respecting their character and purpose. 
Still it is reasonable to suppose that they represent hinge teeth perhaps similar to 
those of Whitella, especially since they lie just within the line of the hinge. As to the 
lower part of the process, why should it not have supported an internal cartilage? 
