576 THE PALEONTOLOGY OF MINNESOTA. 



[Plethocardla umbonatu. 



The shells of this genus present considerable external resemblance to those 

 of Whitella, Ulrich. As a rule they will probably prove shorter, more erect and 

 comparatively more ventricose. I believe also that Whitella offers closer affinities 

 than any other genus yet known, and I can see that it may prove difficult in some 

 cases to distinguish species of the two genera when the internal characters are not 

 available. Of course such difficulties cannot obtain when the diagnostic characters 

 of the hinge are preserved, since the strong subrostral process of Plethocardia is too 

 marked a feature to be overlooked in comparing the two genera. Good casts of the 

 interior even are easily distinguished by the presence of the small lobe beneath and 

 in front of the beaks of Plethocardia, the muscular impressions being very much less 

 distinct in the casts of Whitella. In the posterior part of the hinge, however, as 

 well as in other respects, the two genera are practically the same. 



It seems to me more than doubtful that Plethocardia belongs to the family 

 Megalpdontidce. A general resemblance to those heavy and strongly-hinged Devonian 

 and Triassic shells, which are included in the family by Zittel, may at first strike 

 one, but a critical comparison brings out too many important differences. I adopted 

 the above provisional arrangement chiefly that attention may be directed to the 

 genus as a possible progenitor of a remarkable family of shells. 



PLETHOCAKDIA UMBONATA Ulrich. 



PLATE XL, FIGS. 22-24. 



Plethocardia umbonata ULRICH, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 244. 



Shell about 25 mm. in length, strongly ventricose, obliquely subovate in a side 

 view, widest posteriorly; beaks large, very prominent, inrolled; umbonal ridge angu- 

 lar, traceable to the postero-basal margin; cardinal slope narrow, rather sharply 

 defined, concave. Anterior end very short, nearly ventrical, sharply rounded above; 

 dorsal margin arcuate, graduating into the posterior curve; the latter is produced 

 slightly in the lower part and accelerated as it turns into the broadly convex ventral 

 margin. Surface marked with concentric lines of growth, some of them strong. 



Escutcheon narrow, extending backward from the beaks nearly to the posterior 

 extremity of the hinge. Subrostral cardinal process large, projecting obliquely for- 

 ward from the lower side of the hinge, with one large depression (? internal cartilage 

 pit) in the lower half and several smaller ones (? teeth sockets) above. A strong, 

 ridge-like thickening of the shell, probably representing either a postero-lateral 

 tooth or the support of an internal ligament, occurs just within the postero-cardinal 

 margin. Anterior adductor muscular scar situated in a cup-like depression formed 

 by a curved ridge which proceeds from the under side of the cardinal process and 



