272 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
fine rows of small circular to quadrangular pits. Three to four rows of pits occu- 
pying 1 mm. at the front margin. Pits crudely concentric in arrangment but often 
upset by intercalations offsetting the pits enough to make one or more rows of 
pits appear opposite the crossbars in other rows. 
Pedicle valve unevenly convex in lateral profile with the greatest convexity 
in the umbonal and median regions; anterior third flattened. Apex located about 
one-fourth to one-third the length anterior to the brachial beak or about one- 
fifth to one-sixth the length anterior to the posterior margin. Foramen short, 
semielliptical, about 4 mm. wide on the posterior margin and 3 to 4 mm. long, 
or in other words one-fifth to one-sixth the length. Umbonal region rounded, 
posterior slope steep, anterior slope long and flat, moderately steep. Lateral 
slopes flattened, moderately steep. 
Brachial valve moderately convex in lateral profile but slightly more convex 
in anterior profile. Beak small, incurved; umbo narrowly swollen with steep, 
concave but short umbonal slopes. Median region somewhat inflated ; lateral and 
anterior slopes gentle but the lateral ones the steeper. 
Measurements in mm.— 
Length Width 
Length Width foramen foramen Thickness 
Paratype (pedicle valve 109709c).. 16.7 18.6 2.5 4.5 ? 
- (1007070). cc sec Fe eres 18.5 20.3 2.5 5.0 6.0 
Types——Holotype: 109707d; figured paratypes: 109709a,d; unfigured para- 
types : 109707a-c,e,f, 109709b,c,e. 
Horizon and locality—Upper to feet of the Plattin group (Macy formation) 
or Barnhart formation in Ralls County, Mo.: Yaeger Farm, 54 miles southeast 
of New London; Buford Cave on the M. F. Meyer Farm, 2 miles west and 4 
mile north of New London; Conns Ford in the bed of Salt River about 4 miles 
northwest of New London in NW. corner sec. 33, T. 56 N., R. 5 W. 
Discussion—An examination of the figures of T. huronensis Billings shows 
that the Ralls County specimens under discussion are not conspecific. In the first 
place T. huronensis is a small species about 4 inch or slightly more in length and 
4 inch or less in width. The pedicle valve is strongly concave, which incidentally 
may not be a faithful representation of the true condition of the species. The 
foramen of T. huronensis is figured as elongate, a deep and narrow gash in the 
posterior half of the shell that extends to the apex. These features combined with 
the more elongate form show clearly that the Ralls County specimens are not 
correctly identified with T. huronensts. 
Comparison of the Missouri specimens with T. ottawensts is more difficult be- 
cause only the brachial valve is known with certainty. The pedicle aperture of 
the pedicle valve figured by Hall and Clarke is a doutbful identification and can- 
not be relied upon. The brachial valve as indicated by Billings is a large shell, 
far larger than any of the Ralls County specimens. The ornamentation indicated 
by Billings is that of quadrangular pits opposed by the crossbars of adjacent 
series. The probability is that these are too regularly arranged in this fashion in 
the drawing. The few specimens of this species available for comparison show 
