LAMELLIBRANCHIATA. 5 
Whitella concentrica.] 
perfect material can be compared other differences will become apparent, especially 
in their hinges and muscular impressions, these parts appearing to be somewhat 
stronger in the Trenton shales species. 
Formation and locality.—Middle third of the Trenton shales, Minneapolis and St. Paul, Minnesota. 
WHITELLA conceNTRICA Ulrich. 
PLATE XLI, FIGS. 2 and 3. 
Whitella concentrica ULRICH, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 247. 
Shell rather beneath the medium size, oblique, ventricose, widest posteriorly, 
trapezoidal; beaks large, prominent, incurved; umbones full, with a sharply rounded 
ridge or line of gibbosity extending backward from the beaks to the posterior 
extremity of the shell. Cardinal and posterior slopes shghtly concave. Anterior 
end short, narrowly rounded; ventral edge very gently convex; posterior end pro- 
duced and sharply rounded in the lower half, more gently convex and sloping rapidly 
forward above, merging gradually into the curve of the dorsal side. Hinge line about 
half as long as the shell, with the edge inflected so as to form a narrow escutcheon, 
extending but little, if at all, in front of the beaks. Internal ligamental supports 
leave a distinct impression on each side of the postero-cardinal margin in casts of 
the interior. Anterior muscular scars distinct though faintly impressed, situated in 
the antero-dorsal angle. Surface of casts, especially in the lower and posterior parts, 
marked with fairly distinct, rounded, concentric folds. 
The concentric undulations are stronger in this species than in any other known 
tome. It is shorter than W. pracipta, more ventricose than W. compressa, and has 
much fuller umbones than W. obliquata. In W. scofieldi the surface is not undulated, 
the anterior end is subangular above, and the umbonal ridge sharper. 
Formation and locality.—Middle third of the Trenton shales at Minneopolis, Minnesota. 
WHITELLA RUGATINA, 7. SD. 
PLATE XLI, FIG. 1. 
Shell subovate or obscurely trapezoidal, moderately gibbous, with well shaped 
and prominent umbones and strongly incurved beaks situated one-fourth of the 
length of the shell behind the anterior extremity. Umbonal ridge moderate, rounded 
except immediately behind the beaks. Escutcheon narrow, not extending in front 
of the beaks. Surface of casts marked with numerous, rather small, concentric 
furrows, which in parts may be quite regular, but in others are thrown into bundles 
so as to produce? obscure] undulations. Hinge unknown, muscular and_ pallial 
impressions very faint. 
