PLATE LXXIII. 243 
ACROTRETA SOCIALIS von Seebach—Continued. 
Figures 4c, 4c’, 4c’’, and 4c’. Top, back, side, and front views of a cast of the interior of a ventral valve, showing 
very strong cardinal scars and main vascular sinuses, associated with the shells represented by figures 
4and 4a. U.S. Nat. Mus. Cat. No. 35286c. 
4d. Exterior and side views of a dorsal valve associated with the shells represented by figures 4and 4a. U.S. 
Nat. Mus. Cat. No. 35286d. 
4e. Cast of the interior of dorsal valve associated with the shells represented by figures 4 and4da. U.S. Nat. 
Mus. Cat, No. 35286e. 
ACROTRETA SABRINE (Callaway) (p. 702). 
Figures 5, 5’, and 5’. Top, side, and back views of a cast of part of the interior of a ventral valve. U.S. Nat. Mus. 
Cat. No. 57102a. 
5a, 5a’, and 5a’’. Top, side, and back views of a partly compressed ventral valve. U.S. Nat. Mus. Cat. 
No. 57102b. 
5b. Cast of the interior of a dorsal valve. Type specimen, figured by Callaway [1877, Pl. XXIV, fig. 12] as 
Obolella sabrine. Collection of Dr. Charles Lapworth. 
de. Exterior of a dorsal valve. U.S. Nat. Mus. Cat. No. 57102c. 
5d. Cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 57102d. 
The specimens represented are all from Locality 304h, Upper? Cambrian Shineton shales, Mary Dingle, South 
Shropshire, England. 
ACROTRETA NICHOLSONI Davidson (p. 696). 
[Pl]. LX.XIII, figs. 1, 1a-k.] 
Fieures 6, 6a, and 6b. Top, side, and back views of cast of ventral valve from Locality 304h, Upper ? Cambrian Shine- 
ton shales, Mary Dingle, South Shropshire, England. U.S. Nat. Mus. Cat. No. 5213la. 
