PLATE C. 
a. Cardinal area. , d’. Cast of pseudocruralium. 
b. Dental sockets. « e. Adductor scars. 
ce. Crura. r. Diductor scars. 
er. Pseudocruralium. t. Teeth. 
d. Deltidium. x. Flexure line. 
Nisusia FEsTINATA (Billings) (p. 727). 
(Text fig. 6, p. 299.] 
Ficure 1. Exterior of ventral valve, drawn from mold taken in a natural cast. U.S. Nat. Mus. Cat. No. 52288a. 
Ja. Cast of ventral valve from Locality 25a, Lower Cambrian limestone 2 miles (3.2 km.) east of Swanton, 
Franklin County, Vermont. U. S. Nat. Mus. Cat. No. 15358a. Specimen figured by Walcott 
[1886b, Pl. VII, fig. 7]. 
1b, 1b’, and 1b”. Top, side, and back views of cast of transverse ventral valve. U.S. Nat. Mus. Cat. No, 
52288b. 
le and le’. Top and side views of cast of elevated ventral valve, with strongly marked coste. U.S. Nat. 
Mus. Cat. No. 52288c. 
id and 1d’. Top and back views of cast of ventral valve, an intermediate form between those represented 
by figures 1b and lc. U.S. Nat. Mus. Cat. No. 52288d. 
1d’’. Side view of ventral valve, with finer costz than that represented by figure 1d. U.S. Nat. Mus. Cat. 
No. 52288e. 
le, le’, and le’. Top, side, and back views of a cast taken in a natural mold of a ventral valve, in which 
the area slopes forward. U.S. Nat. Mus. Cat. No. 52288f. 
lf and 1f’. Top and side views of a cast of a dorsal valve. U.S. Nat. Mus. Cat. No. 52288g. 
lg. Cast of the interior of a large dorsal valve. The reference of the markings at “‘e’’ as adductor scars is 
doubtful. U.S. Nat. Mus. Cat. No. 52288h. 
1h and 1h’. Top and back views of a small ventral valve, from Locality 26, Lower Cambrian sandstone east 
of Highgate Springs, Franklin County, Vermont. U. 8. Nat. Mus. Cat. No. 15362a. Specimen 
figured by Walcott [1886b, Pl. VII, figs. 7a—b]. 5354 
liand li’. Top and side views of cast of a small transverse dorsal valve. U.S. Nat. Mus. Cat. No. 522881. 
1j. Interior of a ventral valve drawn from a cast of a natural mold. U.S. Nat. Mus. Cat. No. 52288}. 
With the exception of specimens shown by figures la and 1h, all of the specimens represented by figures 1, la—j 
are from Locality 49, Lower Cambrian decomposed arenaceous limestones on Codorus Creek, below Meyer’s mill, near 
Emigsville, Pennsylvania. 
Ficures 2, 2’: 2a, 2a’; and 2b, 2b’. Top and side views of three ventral valves from Locality 20, Lower Cambrian 
conglomeratic limestones at Bic, Province of Quebec, Canada. U.S. Nat. Mus. Cat. Nos. 52291a, 
52291b, and 52291c, respectively. 
2e, 2c’, and 2c’’. Top, posterior, and side views of a ventral valve, with point of the apex broken off. Asso- 
ciated with the specimens represented by figures 2, 2a—b. U.S. Nat. Mus. Cat. No. 52291d. 
Nisusta ALBERTA (Walcott) (p. 726). 
Figure 3. Top and side view of acast of a ventral valve. It is attached to the dorsal valve shown in figure 3a. 
U.S. Nat. Mus. Cat. No. 24070a. The type specimen is represented by figures 3, 3a—b. 
3a. View of a cast of a dorsal valve, and the area of the ventral valve shown in figure 3.~ U.S. Nat. Mus.. Cat- 
No, 24070a. 
3b. Posterior view of the ventral and dorsal valves represented by figures 3 and 3a. U.S. Nat. Mus. Cat, 
No. 24070a. 
3c. Cast of exterior surface showing ribs and stout bases of spines. U.S. Nat. Mus. Cat. No. 24070b. Copied 
from Walcott [1908c, Pl. I, fig. 4]. 
3d. Interior of the largest shell seen. Geol. Survey Canada. Copied from Walcott [1908c, Pl. I, fig. 4a]. 
The specimens represented are all from Locality 14s, Middle Cambrian Ogygopsis zone on Mount Stephen, above 
Field, British Columbia. 
NISUSIA FESTINATA TRANSVERSA (Walcott) (p. 729). 
Frcures 4and 4a. The two specimens on which the species was founded. U. 8. Nat. Mus. Cat. Nos. 1536la and 
15361b, respectively. Specimens figured by Walcott [1886b, Pl. VII, figs. 5 and 5a] as Orthisina 
transversa. Figure 4 represents the type specimen. 
4b. Interior of a ventral valve. U.S. Nat. Mus. Cat. No. 52298a. 
The specimens represented are all from Locality 25, Lower Cambrian sandstone just above Parker’s quarry, 
near Georgia, Franklin County, Vermont. 
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