PLATE LI. 
f. Flexure lines. r’. Area of anterior adductor sear 
h. Central muscle sear. s. Median septum. 
i. Transmedian muscle sear. y. Visceral cavity. 
j. Anterior lateral muscle scar. vs. Vascular sinus. 
1. Outside lateral muscle scar. ivs. Central lateral branches of vascular 
p. Pedicle groove. system. 
r. Area of posterior adductor scar. x. Heart-shaped cavity. 
ELKANIA DESIDERATA (Billings) (p. 562). 
Fictre 1. Interior of a ventral valve, showing reversed area and muscle scars. U.S. Nat. Mus. Cat. No. 51943a. 
la. Cast of interior of a ventral valve. U.S. Nat. Mus. Cat. No. 51943b. 
1b. Cast of interior of dorsal valve, showing main vascular sinuses with branches extended over-ceniral area. 
U.S. Nat. Mus. Cat. No. 51943c. 
ic. Cast of interior of dorsal valve, showing central area defined, but no traces of branches of main vascular 
sinuses. This central area has been interpreted as probably the area of attachment of muscles, by 
authors who have written of the species. U.S. Nat. Mus. Cat. No. 51943d. 
1d. Cast of interior of a dorsal valve in which the visceral area is beautifully defined, but the vascular sinuses 
and the central area included within them are not defined. U.S. Nat. Mus. Cat. No. 51943e. 
The specimens represented are all from Locality 319a, Lower Ordovician shales, at Point Levis, Quebec. 
ELKanta ampreta (Walcott) (p. 562). 
Fictre 2. Exterior of ventral valve. This specimen can not be located and the locality number is not known. The 
figure, however, is a new drawing of the specimen illustrated by Walcott [1884b, Pl. I, fig. 2b] as 
Obolella? ambigua. 
2a. Partly exfoliated dorsal valve, showing the outlines of the visceral area and main vascular sinuses on 
the inner lamellations of the shell. U.S. Nat. Mus. Cat. No. 24554a. Type specimen, figured by 
Walcott [18S4b, Pl. I, fig. 2a] as Obolella? ambigua. 
2b. Interior of a dorsal valve, showing visceral area, muscle scars, and main vascular sinuses. U.S. Nat. Mus. 
Cat. No. 24554b. Specimen figured by Walcott [1884b, Pl. I, fig. 2c] as Obolella? ambigua. 
2c. Cast of the interior of a dorsal valve in which the thickening of the shell within the lines of the main vascu- 
lar sinuses is greater than in any other specimen. The visceral area also extends farther forward 
into the valve. This variation in the forward extension of the visceral area is not infrequent in 
species of Obolus. U.S. Nat. Mus. Cat. No. 24554c. 
The specimens represented by figures 2a—-c are all from Locality 63, Lower Ordovician: limestone, northeast of 
Adams Hill, Eureka district, Nevada. 
Oxsotts (ForDINIA) BELLULUS (Walcott) (p. 428). 
Fictre 3. Top and side views of a partly exfoliated ventral valve irom Locality 7z, Upper Cambrian limestone, 
about 3 miles southeast of Emigrant Pass, Esmeralda County, Nevada. U.S. Nat. Mus. Cat. No. 
519442. 
3a. Interior of a ventral valve, the type specimen, showing central trapezoidal area at x; also main vascular 
sinuses, pedicle groove, etc. U.S. Nat. Mus. Cat. No. 51944a. 
3b and 3c. Casts of interior of ventral valves. U.S. Nat. Mus. Cat. Nos. 51944b and 51944c, respectively. 
3d. Partly exfoliated dorsal valve. U.S. Nat. Mus. Cat. No. 51944d. 
3e. Interior of dorsal valve. U.S. Nat. Mus. Cat. No. 51944e. 
3i. Cast of interior of dorsal valve. U.S. Nat. Mus. Cat. No. 51944i. 
The specimens represented by figures 3a-f are from Locality 7x, Upper Cambrian limestones about 2.5 miles 
southeast of Emigrant Pass, Silver Peak quadrangle, Esmeralda County, Nevada. 
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