PLATE LIV. 
a. Central lateral space of area. i. Transmedian muscle sear. 
a’. Outer lateral space of area. j. Anterior lateral muscle sear. 
e. Trapezoidal area, including central, 1. Outside lateral muscle sear. 
middle lateral, and outside lateral p. Pedicle groove. 
muscle scars. pe. Cast of pedicle opening. 
f. Flexure line of area. v. Visceral cavity. 
h. Central muscle scar. vs. Main vascular sinus. 
OBOLELLA cHROMATICA Billings (p. 591). 
‘Fiaure 1. Ventral valve of the usual form and outline. U.S. Nat. Mus. Cat. No. 14891a. 
la. Posterior view of the specimen represented by figure 1b, showing area and narrow pedicle slit. U. 8. 
Nat. Mus. Cat. No. 14891b. 
1b and lc. Outer and side view of a ventral valve that is less elongate than specimen represented by figure 1 
(see fig. la). U.S. Nat. Mus. Cat. No. 14891b. 
1d. Dorsal valve, in which the structure of the shell is unusually well preserved. U.S. Nat. Mus. Cat. No. 
14891c. 
le. Cast in limestone of the interior of a ventral valve, showing vascular markings and muscle scars. The 
extreme posterior portion is, unfortunately, broken away. The false pedicle furrow is too deep and 
strong. Itis shallow in the specimen. U.S. Nat. Mus. Cat. No. 14891d. 
1f. Interior of a ventral valve that has a narrow thickening of the shell just in front of the area, corresponding 
to the posterior portion of the thickened shell usually present in the ventral valve of Obolella crassa 
beneath the visceral area. (See figs. 2g and 2i.) Geol. Survey Canada. 
lg. Cast in limestone of the interior of a small dorsal valve. U.S. Nat. Mus. Cat. No. 14891e. 
lh. Interior of a dorsal valve. The area is outlined from a specimen in the collection of the Geological Sur- 
vey of Canada. U.S. Nat. Mus. Cat. No. 14891f. Specimen figured by Walcott [1886b, Pl. XI, 
fig. lb]. The specimens represented by figures 1 and la [Walcott, 1886b, Pl. XI] are not redrawn 
in this monograph. 
li. Interior of a dorsal valve drawn from a plaster cast of a limestone cast in the collection of the Geological 
Survey of Canada; cast in the United States National Museum (Cat. No. 14891g). Cast figured by 
Walcott [1886b, Pl. X, fig. 3]; and [189la, Pl. LX-XJ, fig. 3] as Obolella circe. 
The specimens represented by figures 1, la—i are all from Locality 392a, Lower Cambrian limestone at L’Anse au 
Loup, Straits of Belleisle, Labrador. The material in the collections of the Geological Survey of Canada includes 
the specimens upon which Billings founded the species, but closer identification is impossible. 
OBOLELLA CRASSA (Hall) (p. 592). 
[Text fig. 14, p. 299.] 
Ficure 2. Exterior of a ventral valve of the usual form and outline from Locality 27, Lower Cambrian limestone in 
the eastern suburb of Troy, New York. U.S. Nat. Mus. Cat. No. 15348a. 
2a. An unusually elongate ventral valve, with the surface characters well preserved, from Locality 2b, Lower 
Cambrian limestone just north of Beman Park, Troy, New York. U.S. Nat. Mus. Cat. No. 5195la. 
2b. An unusually broad ventral valve associated with the shell represented by figure 2a. U.S. Nat. Mus. 
Cat. No. 51951b. : 
2c and 2d. Exterior and side views of a dorsal valve, associated with the shell represented by figure 2a. U.S. 
Nat. Mus. Cat. No. 51951c. 
2e. Posterior view of a ventral valve associated with the shell represented by figure 2a, showing low area and 
the pedicle slit. U.S. Nat. Mus. Cat. No. 51951d. 
2f. Section of a ventral valve associated with the shell represented by figure 2a, naturally broken near the 
median line. The thickest part of the shell is over the visceral area. U.S. Nat. Mus. Cat. No. 
51951e. 
2g, 2h, and 2i. Interiors of ventral valves illustrating variation in the areas, vascular markings, and muscle 
scars. Note the narrow rounded area and the shallow median furrow in figure 2h; the inner end of 
the pedicle tube is shown on the specimen just beneath the inner edge of the area. Locality 2b, 
Lower Cambrian limestone just north of Beman Park, Troy, New York. U.S. Nat. Mus. Cat. Nos. 
51951, 5195lg, and 51951h, respectively. 
