928 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
STROPHOMENA AUBURNENSIS Fenton 
Plate 257, A, figures 1, 2; plate 263, C, figures 17-22 
Strophomena auburnensis Fenton, Amer. Midland Nat., vol. 11, Nos. 3, 4, p. 148, pl. 9, 
figs. I-3, 1928. 
This species is a large one characterized by important exterior and interior 
details. The pedicle valve is completely concave except for the umbonal region 
which is slightly swollen or flat. The brachial valve is flattened in the posterior 
third when viewed in lateral profile, but the anterior two-thirds is fairly strongly 
convex. The pseudodeltidium is subcarinate like that of S. crassicostellata Cooper. 
The surface is marked by costellae of unequal size, strong costellae setting off 
groups of finer ones, as many as 5 of the finer ones in a group. 
Inside the pedicle valve the muscle area is quite distinctive. Its length and 
width are about equal, and the sides are nearly straight, so that the field is like 
a small square with one angle in the delthyrial cavity, the two lateral ones nar- 
rowly rounded and the fourth directed anteriorly. Inside the brachial valve the 
brachiophores are rudimentary and enclosed in a small cup of adventitious shell ; 
the cardinal process is delicate and the median ridge short. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
Hypotype (117704a)..... 27:3 2 25.6 ? 36.0 35.6 ? 
. CTI7704D) ic. 25.1 ? 32.8 34.9 ? 
“ (TV77OZ es 35.8 34.7 45.0 47.6 7.8 
Types.—Figured hypotypes: 117702, 117704a,b. 
Horizon and locality —Auburn chert in Missouri: $ mile east of Auburn, Els- 
berry (15’) Quadrangle. Platteville formation: On Wisconsin Highway 81, 
2 miles northwest of Platteville, Mineral Point (30’) Quadrangle; 0.7 mile 
northeast of Dickeyville on Wisconsin Highway 151, Grant County. 
Decorah formation (Guttenburg member) in Wisconsin: 0 to 8 feet above 
8-inch blue clay on top of Platteville, Wisconsin Highway 81, 0.1 mile south of 
the Little Platte River, 2 miles northwest of Platteville, Mineral Point (30’) 
Quadrangle. 
Decorah formation (Spechts Ferry member—Glass Rock) in Illinois: 14 miles 
northeast of Dixon, Dixon (15’) Quadrangle. 
Discussion—tThis species is characterized by large size and strong differenti- 
ation of its costae. It is a large and striking species and evidently at times has 
been confused with S. filitexta Hall. 
STROPHOMENA AUBURNENSIS IMPRESSA Raasch 
Strophomena auburnensis impressa Raasch, in SHRocK and RaAscH, Amer. Midland Nat., 
vol. 18, No. 4, pp. 549, 592, pl. 4, figs. 6, 8, 1937. 
Horizon and locality.—Platteville formation (upper Buff) in Wisconsin: At 
Beloit, Rock County; 3 miles south of New Glarus, Green County. 
