,„ , , „. , LAMELLIBEANCHIATA 575 



umbones less tumid and not so prominent. The impressions of the internal liga- 

 ment supports also are very much less distiiftt. W. suhcarinata is not so oblique, 

 shorter and has a longer hinge and narrower escutcheon. 



Formation and ZoeoH*y.— Galena shales near Cannon iFalls, Minnesota. 



Family PMEGALODONTIDtE, Zittel 

 Genus PLETHOCARDIA, IJlrich. 



Plethooardia, Ulbich, 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, Poaterior. 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 hiuge 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 01* 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 pirocess 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? 



