PLATE XVI. 
a. Central lateral space of area. k. Middle lateral muscle scar. 
a’. Outer lateral space of area. 1. Outside lateral muscle scar. 
ce. Trapezoidal area, including central, middle p. Pedicle groove. 
lateral, and outside lateral muscle scars. v. Visceral cavity. 
h. Central muscle scar. vs. Main vascular sinus. 
i. Transmedian muscle scar. x. Heart-shaped cavity. 
j. Anterior lateral muscle scar. 
Oxsotus (LINGULOBOLUS) AFFINIS (Billings) (p. 431). 
Figure 1. Cast of the interior of a ventral valve showing the characteristic viséeral cavity. U.S. Nat. Mus. Cat. 
No. 51679a. 
la. Cast of a portion of the interior of a ventral valve. U.S. Nat. Mus. Cat. No. 51679b. 
lb. Cast of the interior of a ventral valve showing the ae - the deep impressions in which the muscle scars 
are situated. U.S. Nat. Mus. Cat. No. 51679c. 
le. Cast of the interior of a dorsal valve showing the position of the central and anterior lateral muscle scars. 
U.S. Nat. Mus. Cat. No. 51679d.- 
ld. Partly exfoliated dorsal valve showing the area and the varied surface of the inner layers of the shell. 
U.S. Nat. Mus. Cat. No. 51679e. 
ie. Outline of the area of a dorsal valve. U.S. Nat. Mus. Cat. No. 51679f. 
The specimens represented are all from Locality 114b, Lower Ordovician sandstone, 1 mile north of Lance Cove, 
Great Belle Island, Conception Bay, Newfoundland. 
Oxso.tus (LinGULOBOLUS) sPIssus (Billings) (p. 432). 
[Pl. XLH, figs. 3, 3a.] 
Figure 2. Dorsal valve from which the outer layers of the shell have been exfoliated so as to show the outline of the 
visceral cavity and vascular sinuses. U.S. Nat. Mus. Cat. No. 51678 1. Part of the inner surface 
of this specimen is enlarged on Plate XLII, figure 3a. 
29. Cast of the interior of a dorsal valve, on which portions of the shell remain over ‘the muscle scars. U.S. 
Nat. Mus. Cat. No. 51678a. 
2b. Dorsal valve preserving a portion of the outer surface of the shell, which is enlarged in Plate XLII, 
figure 3. U.S. Nat. Mus. Cat. No. 51678b. 
2e. Partly exfoliated dorsal valve showing something of the character and arrangement of the inner layers of 
the shell. U.S. Nat. Mus. Cat. No. 51678c. 
2d. Partly exfoliated dorsal valve illustrating how the impressions of the vascular sinuses and markings are 
preserved on the various layers of the shell. U.S. Nat. Mus. Cat. No. 51678d. 
2e. Inner layers of the central portion of a ventral valve, showing the characteristic trapezoidal area and 
vascular sinuses of Obolus. U.S. Nat. Mus. Cat. No. 51678e. 
2f. Partly exfoliated dorsal valve illustrating peculiarities of the shell not shown by other figures. U.S. Nat. 
Mus. Cat. No. 51678f. 
Qe. Cast of the interior of a dorsal valve with vertical and transverse outline of the convexity of the shell. 
U.S. Nat. Mus. Cat. No. 51678g. 
2h. Partly exfoliated dorsal valve that is more elongate than most specimens. U.S. Nat. Mus. Cat. No. 51678h. 
2i. An unusually well preserved ventral valve, showing the various layers forming the shell. U. S. Nat. 
Mus. Cat. No. 51678i. 
2j. Interior of a ventral valve as shown by the impression made on the laminated inner layers of the shell, 
the outer layers being exfoliated. U.S. Nat. Mus. Cat. No. 51678}. 
2k. Cast of the interior of a ventral valve, showing muscle scars and vascular markings essentially the same 
as those of Obolus. U.S. Nat. Mus. Cat. No. 51678k. 
The specimens represented are all from Locality 114b, Lower Ordovician sandstone, 1 mile north of Lance Cove, 
Great Belle Island, Conception Bay, Newfoundland. 
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