Strophomena and Other Fossils 
55 
wide, but may equal 9 mm. in the same width. The ratio of the 
length of the shell to the width usually varies between 0.70 and 
0.80. 
The muscular area of the pedicel valve is shallow, but the curved 
lateral border is distinct, especially along the exterior edge. 
Anteriorly, this lateral border is deflected slightly forward, pro- 
ducing a median gap. The median ridge traversing the muscular 
area is low and rather narrow anteriorly. There are only faint 
traces of median vascular markings or of any thickening of the 
shell toward the anterior margin. The interior surface, in gen- 
eral, is smooth. 
In addition to the cardinal process, the crural plates, and the 
low median elevation, the interior of the brachial valve may 
show the posterior part of the inner pair of the vascular ridges 
frequently found in species of Strophomena. 
The radiating striae are much coarser than in most species of 
Strophomena. This coarseness is especially noticeable over the 
middle regions of the shell, and even toward the beak, where the 
radiating striae usually are comparatively fine. About 7 to 9 
striae occupy a width of 5 mm. near the margin of the brachial 
valve, while 7 to 10 occur in the same width on the pedicel valve. 
The type specimens of Strophomena planoconvexa were found at 
Cincinnati, Ohio, and here they occur at the base of the Fairmount 
bed. They are found at this horizon as far north as the clay banks 
on the W est Fork of Mill Creek, southwest of Glendale ; 1 mile east 
of Reading; and at Allandale, miles northeast of Madisonville, 
in Hamilton county. Also, northeast of Crescent Springs, along 
the railroad, in Kenton county, Kentucky; and nearly 2 miles 
west of Verona, in the southern part of Boone county. The upper 
horizon of Strophomena along the railroad north of Williamstown, 
in Grant county, Kentucky, consists, of Strophomena planoconvexa 
at the base of the Fairmount, and this horizon, with the same form 
of Strophomena, may be traced as far south as Mason. The upper 
horizon for Strophomena, from Guilford, in Dearborn county, to 
the vicinity of Vevay, in Switzerland county, in Indiana, is approxi- 
mately at the same horizon, the base of the Fairmount, and con- 
tains the same species, Strophomena planoconvexa. 
Along the railroad from Consolation to Benson, in the eastern 
part of Shelby county, Kentucky, the specimens of Strophoinena 
in the Mount Hope and in the lower part of the Fairmount section. 
