72 
IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
to twenty-two strong, simple, subangular plications, separated 
by deep, narrow furrows. Entire surface, to within 3 or 4 mm. 
of the beak, marked by heavy, concentric, lamellose growth lines, 
which are particularly abundant near the anterior margin. The 
species is distinguished by these lines, its pronouncedly curved 
postero-lateral margin, sharply projecting pedicle beak, and its 
sharply angular plications. 
Formation and Locality : Black River (Platteville), Ellsworth, 
Wisconsin. 
Holotype, No. 27,444, Walker Museum. 
RHYNCHOTRBMA CUNEIFORMS sp. nov. 
Plate I, figs. 12-14. 
Shell small, longer than wide, subtriangular. Postero-lateral 
margins nearly straight, meeting in a very acute angle at the 
beak; postero-lateral slopes flattened, but with the plications pres- 
ent. Dimensions of the holotype : Length, 10.6 mm. ; width, 10 
mm. ; thickness, 8 mm;. ; width of sinus, 5.6 mm. 
Pedicle valve moderately convex in the umbonal region ; slightly 
flattened anteriorly, with an abrupt convexity near the anterior 
margin. The broad, shallow sinus begins about 5 mm. from the 
beak, and is occupied by three to four strong plications. Beak 
long, sharp, slightly incurved. 
Brachial valve deeper and more convex than the pedicle ; flat- 
tened along the postero-lateral margins, and abruptly convex 
anteriorly. Beak short and abruptly incurved into the delthyrium ; 
mesial fold originates anterior to the middle of the shell and 
bears four or five strong plications. Surface of both valves 
marked by thirteen to fifteen strong, subangular plications, which 
are reduced in size along the postero-lateral area. Specimens 
from the Decorah shale are larger and more coarsely plicated than 
those from the Plattin. 
Formations and localities : Plattin, Ste. Genevieve Co. ; De- 
corah Shale, near Ste. Genevieve, Missouri. 
Holotype, No. 25,639, Walker Museum. 
RHYNCHOTRBMA DBCORAHBNSB sp. nov. 
Plate I, figs. 9-11. 
Shell small, broadly subtriangular ; as wide as, or wider than, 
long. Postero-lateral slopes without vertical flattening. Dimen- 
