LAMELLIBRANCHIATA. 569 



Whitella ooncentrioa.] 



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. concenteica TJlrich. 



PLATE XLI, FIGS. 2 and 3. 



Whitella concentrica Uleich, 1892. Nineteenth Ann. Eep. G-eol. 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 slightly 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 pesterior parts, 

 marked with fairly distinct, rounded, concentric folds. 



The concentric undulations are stronger in this species than in any other known 

 - to me. It is shorter than W. prcecipia, more ventricose than W. compressa, and has 

 much fuller umbones than W. ohliquata. 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 euqatina, n. sv. 



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 

 fiirfows, 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>ery faint. 



