PLATE LXXVI. 251 
ACROTHYRA PROAVIA (Matthew)—Continued. 
Figure 3f. Ventral valve. U.S. Nat. Mus. Cat. No. 52052d. 
3g. Posterior view of false area of a specimen associated with the specimen represented by figure 3. Univer- 
sity of Toronto, Canada. 
3h. Posterior view of false area of a much broader shell than that represented by figure 3g. U.S. Nat. Mus. 
Cat. No. 52052e. 
3i, 3k, and 31. Casts of ventral valves showing cast of visceral cavity. U.S. Nat. Mus. Cat. Nos. 52052f, 
52052g, and 52052h, respectively. 
3m. Interior of a ventral valve. U.S. Nat. Mus. Cat. No. 52052i. 
3n. Interior of a ventral valve with the apex broken away. U.S. Nat. Mus. Cat. No. 52052]. 
30. Two ventral valves illustrating effect of distortion by compression. U.S. Nat. Mus. Cat. No. 52052k. 
3p. Interior of a dorsal valve associated with many specimens of this species, which varies in outline and 
internal markings from all other specimens. This may be owing to elongation and compression in 
the matrix. U.S. Nat. Mus. Cat. No. 520521. 
3q. Interior of dorsal valve. U.S. Nat. Mus. Cat. No. 52052m. 
3r and 3s. Casts of interior of dorsal valves. U.S. Nat. Mus. Cat. Nos. 52052n and 520520, respectively. 
3t. Interior of dorsal valve. U.S. Nat. Mus. Cat. No. 52052p. 
3u. Exterior and side outline of a dorsal valve. U.S. Nat. Mus. Cat. No. 52052q. 
With the exception of figures 3, 3c, 3d, and 3g, all of the specimens represented by figures 3, 3a—u are from Locality 
10q, Middle Cambrian sandstone below the waterfall in Division E2b of Matthew’s [1903, p. 21] Etcheminian, Dugald 
Brook, Indian River, eastern Cape Breton, Nova Scotia. 
ACROTHYRA MINOR Walcott (p. 717). 
[Text fig. 59, p. 717.] 
Figures 4 and 4’. Top and side views of a nearly perfect ventral valve, the type specimen. U.S. Nat. Mus. Cat. 
No. 52050a. 
4a and 4a’. Top and side views of a cast of the interior of a ventral valve that had a large apical callosity. 
U.S. Nat. Mus. Cat. No. 52050b. 
4b, 4b’, and 4b’. Top, back, and side views of a cast of the interior of a ventral valve. U.S. Nat. Mus. 
Cat. No. 52050c. 
The specimens represented are all from Locality 5b, Middle Cambrian Langston limestone near the mouth of 
Twomile Canyon, 2 miles (3.2 km.) southeast of Malade, Oneida County, Idaho. 
