PLATE L. 
a. Central lateral space of area. j. Anterior lateral muscle scar. 
a’. Outer lateral space of area. k. Middle lateral muscle sear. 
b. Boss (see text description, p. 571). 1. Outside lateral muscle scar. 
e. Trapezoidal area, including central, middle mr. Median ridge. 
lateral, and outside lateral muscle scars. p. Pedicle groove. 
f. Flexure line of area. ps. Parietal band. 
g. Umbonal muscle sear. s. Median septum. 
h. Central muscle scar. vs. Main vascular sinus. 
i. Transmedian muscle scar. x. Heart-shaped cavity. 
Brcra GEMMA (Billings) (p. 569). 
Fraures 1 and 1b. Exterior and side view of a ventral valve with the outer surface partly removed by erosion. 
Locality 27, Lower Cambrian limestone near Troy, Rensselaer County, New York. U.S. Nat. Mus. 
Cat. No. 15349a. 
la. Exterior of a ventral valve associated with the specimen represented by figure 1, preserving the interrupted 
radiating strive of the outer surface. U.S. Nat. Mus. Cat. No. 15349b. 
le and 1c’. Exterior of two dorsal valves, associated with the specimen represented by figure 1. U.S. 
Nat. Mus. Cat. Nos. 15349¢ and 15349d, respectively. Specimen figured by Walcott [1886b, Pl. X, 
fig. 2e] as Obolella gemma. 
1d. Posterior portion of a ventral valve associated with the shell represented by figure 1, on which the con- 
centric and radiating lines are very strong (see fig. le). U.S. Nat. Mus. Cat. No. 15349e. 
le. Outer surface of a ventral valve from Locality 2b, Lower Cambrian limestone near Beman Park, Troy, 
New York, greatly enlarged. This may possibly be the surface of Bicia whiteavesi. U.S. Nat. Mus. 
Cat. No. 51900a. 
lf. Natural cast of interior of ventral valve from Locality 20, Lower Cambrian limestone bowlders at east 
entrance to Bic Harbor, Quebec. U.S. Nat. Mus. Cat. No. 14889d. 
lg. Natural cast of interior of ventral valve associated with the specimen represented by figure le, at Troy, 
New York. U.S. Nat. Mus. Cat. No.51900b. Another specimen (Cat. No. 51900c) was used to aid 
in restoring certain portions of this figure. 
th, li, and 1k. Interiors of ventral valves showing variation in area, thickening of shell, etc. They are 
associated with the specimen represented by figure 1f, at Bic Harbor, Canada. U.S. Nat. Mus. Cat. 
Nos. 14889c, 14889b, and 14889a, respectively. The specimens represented by figures li and 1k were 
illustrated by Walcott [1886b, Pl. X, figs. 2b and 2d, respectively] as Obolella gemma. The specimen 
illustrated by Walcott [1886b, Pl. X, fig. 2] is not figured in this‘ monograph. 
1j. Interior of ventral valve, with thick shell growth, from Locality 27, Lower Cambrian limestone near 
Troy, New York. U.S. Nat. Mus. Cat. No. 15349e. Specimen figured by Walcott [1886b, Pl. X, 
fig. 2c] as Obolella gemma. : 
11. Natural cast of a distorted dorsal valve associated with the specimen represented by figure le, at Troy. 
New York. U.S. Nat. Mus. Cat. No. 51900d. 
Im. Interior of a dorsal valve associated with the shell represented by figure 1f at Bic Harbor, Canada. U.S. 
Nat. Mus. Cat. No. 14889f. 
In. Interior of dorsal valve from Locality 2b, Lower Cambrian limestone near Beman Park, Troy, New York. 
U.S. Nat. Mus. Cat. No. 51900e. This drawing is based on one fairly well preserved interior, but 
the anterior lateral muscle scars and some other details are taken from five other associated specimens 
(Cat. Nos. 51900f-). 
Bicta wuirEAvest Walcott (p. 570). 
Ficures 2, 2a, and 2b. Interiors of ventral valves, showing transition from short form of ventral valve of B. gemma, 
figure 1k, to the typical ventral valve of B. whiteavesi, figure 2c. U.S. Nat. Mus. Cat. Nos. 35260a, 
35260b, and 35260c, respectively. 
2c. Interior of a ventral valve, the type specimen. U.S. Nat. Mus. Cat. No. 35260d. 
2d. Natural cast of the interior of the specimen represented by figure 2c. U.S. Nat. Mus. Cat. No. 35260e. 
2e. Interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 352608. 
The specimens represented are all from Locality 2b,-Lower Cambrian limestone near Beman Park, Troy, New 
York. 
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