760 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
of muscle field. Vascula media short and thick, bifurcating into a larger inner 
branch and a small, shorter outer branch. Interarea curved, longer than the one 
on the brachial valve, hypercline. Pseudodeltidium short. 
Brachial interior with strongly bilobed visceral disk, lobes triangular to 
rounded and the seat of attachment of the adductor muscles. Lobes with ele- 
vated rims on inside ; rims extending nearly to cardinal process. Lobes separated 
by a high, thin median septum extending to about the middle. Brachial processes 
short and small; cardinal process with short, stout shaft bearing 3 ridges on the 
myophore face, a median elevated ridge and lateral lower ridges. 
Genotype.—Bilobia hemisphaerica Cooper, new species. 
Discussion—Bilobia can be recognized by its nearly hemispherical brachial 
valve and the bilobed character of the visceral disk. The genus is most like 
Leptellina in all its characters but differs in its greater sphericity, generally larger 
pedicle muscle field, and in the bilobed character of the visceral disk. The latter 
is brought about by lack of development of a prominent median septum. 
In the European faunas Bilobia occurs in the Kuckers formation of Estonia 
where it is represented by B. musca (Opik). It occurs in the Girvan fattna of 
Scotland where it is known as Bilobia etheridgei (Davidson). In the Appa- 
lachians this genus has been taken from the lower part of the Edinburg (Cyrto- 
notella zone) Shippensburg, and Oranda formations. It is also known in the 
Rysedorf conglomerate. It extends from the Oranda into the base of the Mar- 
tinsburg shale in Virginia where it occurs with elements of the Salona fauna. 
BILOBIA HEMISPHAERICA Cooper, new species 
Plate 192, C, figures 16-20; plate 193, C, figures 10-15; plate 193, D, figures 16-36; 
plate 194, F, figures 31-36 
Plectambonites pisum BASSLER (not Ruedemann), Cambrian and Ordovician: Maryland 
Geol. Surv., p. 253, 1919. 
Shell small, almost hemispherical in lateral profile; wider than long with the 
greatest width at the hinge; cardinal extremities auriculate. Lateral and anterior 
margins irregular; front margin varying from broadly rounded to nasute. Pos- 
terior swollen part of pedicle valve marked by 3 strong costellae that stand out 
over all the others. At the front margin about 17 strong costellae occur in a 
matrix of fine lines, 8 or 9 of the latter in a space of 1 mm. The costellae of the 
brachial valve seem to have fewer of the large costellae, but this may be because 
of faulty preservation. Many of the specimens in the anterior parts of both valves 
are marked by concentric lamellae like those of the associated Bimuria lamellosa. 
Pedicle valve nearly hemispherical in lateral profile and narrowly convex in 
anterior profile. Umbo and median part of valve swollen; all slopes steep but 
more particularly the lateral ones which may be as steep or steeper than the an- 
terior slope. Posterolateral area leading to the cardinal extremities narrowly 
rounded. Interarea short, curved, acutely hypercline in position. Pseudodel- 
tidium short in the middle part, extended laterally as an elevated rim on the 
sides of the delthyrium. 
