762 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Types.—Hypotypes: 84258a-c. 
Horizon and locality——Kuckers formation (3ca): At Kohtla in Estonia. 
BILOBIA PISUM (Ruedemann) 
Plectambonites pisum RUEDEMANN, New York State Mus. Bull. 49, p. 10, pl. 1, figs. 8, 9, 
12-15, 17-20. (10?, 11?, 16?), 1901.—BASSLER, Cambrian and Ordovician: Maryland 
Geol. Surv., p. 253, pl. 48, figs. 3-6 (1, 2?), 1919, repeats Ruedemann’s description and 
figures. 
This species has been widely but usually erroneously identified, because it has 
the form and sculpture of a number of other stocks of plectambonitids such as 
P. scissa Salter, P. transversalis (Wahlenberg), and P. gibbosus Winchell and 
Schuchert. It is another interesting case of heterochronous homeomorphy. The 
species as figured by Ruedemann appears to be composite and perhaps contains 
more than a single genus. Wide and narrow forms are represented which are 
probably conspecific, but the specimen represented by figures Io and 11 has very 
different proportions from those given in the description and probably should 
be excluded from the species. 
In comparison with Bilobia hemisphaerica Ruedemann’s figures and specimens 
from the gray crystalline limestone (Rysedorf conglomerate) in the National 
Museum indicate that B. pisum is generally more strongly convex than the 
Oranda species and the pedicle muscle area is smaller. One of the Rysedorf speci- 
mens (fig. 17) has a curved furrow on the inside just anterior to the cardinal 
extremity. This feature also appears on specimens from Guilford Springs, Pa. 
It is less marked in the strongly rotund forms. 
B. pisum comes from the black and gray crystalline limestone pebbles of the 
Rysedorf conglomerate according to Ruedemann. 
BILOBIA VIRGINIENSIS Cooper, new species 
Plate 192, A, figures 1-5; plate 192, B, figures 6-15; plate 195, B, figures 6-8 
Small, tumid, semicircular in outline, wider than long and with the greatest 
width in the posterior half; sides moderately oblique to nearly straight. Anterior 
margin narrowly rounded. Surface marked by costellae of unequal size, valves 
differently marked. Pedicle valve with about 15 strong, threadlike costellae on 
the front margin but only a few of them reaching the umbo, spaces between larger 
costellae marked by a mat of fine costellae. Brachial valve covered by a mat of 
fine, nearly even costellae. 
Pedicle valve strongly convex in anterior and lateral profiles; posterior slope 
steep and rounded; anterior slope longer but less steep than the posterior one; 
lateral slopes long and steep. Umbo moderately swollen; median region tumid. 
Interarea short, anacline. 
Brachial valve deeply concave, with the maximum concavity just anterior to 
the middle; anterior sulcate; anterior margins strongly bent in the direction of 
the brachial valve. Cardinal extremities with short, narrow grooves to the ex- 
terior. Interarea short, hypercline. Lobes of visceral disk long and narrow. 
