PLATE LXXVII. 
el. Cardinal scars. s. Median septum. 
fp. False pedicle groove. vs. Main vascular sinus. 
h. Central scars. x’’. Cast of apical callosity. 
ACROTRETA CONCENTRICA Walcott (p. 681). 
Figures 1, 1’, and 1”. Top, side, and back views of the exterior of a ventral valve, the type specimen. U.S. Nat. 
Mus. Cat. No. 35266a. 
la and la’. Top and side views of a partial cast of the interior of a ventral valve, showing the base of strong 
vascular sinuses and cardinal scars. U.S. Nat. Mus. Cat. No. 35266b. 
1b. Cast of a compressed dorsal valve. U.S. Nat. Mus. Cat. No. 35266c. 
The specimens represented are all from Locality 96x, Middle Cambrian limestone 1 mile (1.6 km.) north of Adairs- 
ville, 15 miles (24.2 km.) northwest of Cartersville, Cartersville quadrangle (U. S. Geol. Survey), Bartow County, 
Georgia. 
ACROTRETA BAILEYI Matthew (p. 676). 
Ficurgs 2, 2’, and 2’. Top, side, and back views of a slightly distorted ventral valve. Type specimen, figured by Mat- 
thew [1886, Pl. V, figs. 13c and 13d]. This specimen has been repeatedly figured; see page 676 for 
record. 
2a. Cast of a distorted dorsal valve. Specimen figured by Matthew [1886, Pl. V, fig. 13]. The specimen has 
been repeatedly figured; see page 676 for record. 
2b and 2c. Casts of the interior of distorted dorsal valves. 
2d. Interior of a compressed dorsal valve. & 
The specimens represented are all from Locality 300, Middle Cambrian shaly sandstones, possibly of Division 1d 
of Matthew, on Long Reach, St. John River, Kings County, New Brunswick. They are in the collection of the Uni- 
versity of Toronto, Canada. 
ACROTRETA SEEBACHI Walcott (p. 710). 
Ficures 3, 3’, and 3’. Top, back, and side views of a cast of the interior of a ventral valve from Locality 8x, 
passage beds between the Upper Cambrian and the Ordovician, Ceratopyge limestone at the cement 
works at Slemmestad, about 3 miles (4.8 km.) southwest of Christiania, Norway. U.S. Nat. Mus. 
Cat. No. 52172a. This is the type specimen. 
3a, 3a’, and 3a/’. Top, side, and back views of the exterior of a ventral valve associated with the specimen 
represented by figure 3. U.S. Nat. Mus. Cat. No. 52172b. 
AOROTRETA PARVULA (Wallerius) (p. 699). 
Ficures 4, 4’, and 4”. Top, back, and side views of ventral valve from Locality 310i, passage beds between the Mid- 
dle and Upper Cambrian, in limestone at Borgholm, Oeland Island, Sweden. U.S. Nat. Mus. Cat. 
No. 35288a. 
4a. Exterior of dorsal valve, associated with the specimen represented by figure 4. U.S. Nat. Mus. Cat. 
No. 35283b. 
ACROTRETA GEMMULA Matthew (p. 686). 
[Pl. LXVI, figs. 3, 3a-c.] 
Figures 5, 5’, and 5’. Top, back, and side views of the exterior of a ventral valve from Locality 10p, Middle Cam- 
brian sandstones of Division E2b of Matthew’s Etcheminian on Dugald Brook, Indian River, Cape 
Breton, Nova Scotia. U.S. Nat. Mus. Cat. No. 52088a. 
5a, 5a’, and 5a/’. Top, back, and side views of a ventral valve showing the cast of the surface of the apical 
callosity. Locality 10p’’, Middle Cambrian sandstone on the small brook on the hill between the 
bridge over Indian River and McPhees Brook, Cape Breton, Nova Scotia. U.S. Nat. Mus. Cat. 
No. 52088b. 
5b, 5b’, and 5b’. Top, back, and side views of a ventral valve associated with the specimen represented 
by figure 5, showing cast of base of vascular sinuses, surface of apical callosity, and pedicle tube. 
U.S. Nat. Mus. Cat. No. 52088c. 
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