994 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
but these are associated with rounder specimens. In some localities the subrec- 
tangular specimens predominate. It is possible that refinement of the stratigraphy 
in the Plattin group and Decorah formation of Missouri will show the predomi- 
nantly subrectangular forms to have a different level from that of the rounder 
ones. 
The type specimen of P. subaequata is evidently a young specimen, but it is a 
nearly circular form with numerous swollen costellae concentrated in the median 
region of both valves. The stratigraphic level of this specimen is evidently the 
Guttenberg member of the Decorah formation. According to Hall (1847, p. 118) 
“The species occurs at Mineral Point (Wisconsin), in the Blue limestone asso- 
ciated with Leptaena sericea, Orithis testudinaria, Delthyris lynx and other Tren- 
ton limestone fossils.” A specimen from the Hall collection (123234) now in 
the National Museum is a limestone plate containing a pedicle valve of P. subae- 
quata having dimensions identical to those of the type specimen. In addition two 
valves of Sowerbyella are present. 
Pionodema subaequata is a large species for the genus and may therefore be 
compared with P. minnesotensis and P. circularis. It differs from the former 
in not having the shouldered outline, although the hinge of P. subaequata is 
very wide. Pionodema subaequata has a greater proportion of swollen costellae 
than P. minnesotensis. Pionodema circularis is readily distinguished from 
P. subaequata by its strongly rounded contours and finer costellae. 
PIONODEMA SULCATA Cooper, new species 
Plate 155, D, figures 19, 20; plate 155, E, figures 21-30 
Shell of about medium size for the genus, slightly wider than long with well- 
rounded sides and front margin. Anterior commissure nearly rectimarginate. 
Costellae fine, subequal except on the posterolateral extremities, numbering about 
3 to the millimeter at the front margin. 
Pedicle valve about as deep or slightly deeper than the brachial valve; lateral 
profile uneven, most convex just anterior to the umbo and flattened in the an- 
terior half; anterior profile somewhat narrowly swollen in the median region 
and with long and moderately steep slopes. Beak elevated, slightly incurved ; 
umbo narrowly swollen, the swelling continued anteriorly as a poorly defined 
narrow fold to about the middle of the valve; lateral slopes long and moderately 
steep and gently convex. Interarea curved, apsacline. 
Brachial valve moderately convex in lateral profile, most convex in the me- 
dian region. Anterior profile broadly convex and with the median portion slightly 
flattened and depressed medially by a narrow sulcus. Umbo sulcate, the sulcus 
shallow and narrow but extending anteriorly at least to the middle of the valve 
or beyond. Median region inflated; anterior slope moderately steep; lateral 
slopes short and steep. Cardinal process large, shaft long; brachiophore plates 
flaring. 
Measurements in mm.—Holotype, length 10.7, brachial length 10.0, width 
11.6, hinge width 8.1, thickness 6.5. 
