126 
Aug. F. Foerste 
from the Orthoceras fosteri horizon upward for a distance of 5 to 
10 feet. Some of these specimens are very large, frequently 
reaching a width of 22 mm. These specimens show the oblique 
wrinkling along the hinge-line, the so-called vertical wrinkles 
when examined by transmitted light, and the external thickening 
of the shell due to the presence of more or less discrete or con- 
joined films, as described in the preceding lines under Plecto/m- 
bonites rugosa from the Eden beds. Plectamhonites occurs also 
between the lower Hebertella insculpta horizon and the strata 
containing Strophomena nutans and Strophomena neglecta, in 
the lower part of the Blanchester division. It is abundant in the 
lower part of the Liberty member, frequently ranging upward 
through half of the member. It is seen again in the lower part 
of the Whitewater member, from 7 to 13 feet above the base, but 
here it is rather scarce and small. 
Various attempts to distinguish the numerous specimens found 
in the Waynes ville and Liberty members from those found in the 
lower and middle Eden have not proved very successful. As a 
rule, the delthyrium of the Liberty and Waynesville specimens 
is narrower, the sides forming an angle of about 67 degrees, while 
in the Eden specimens this angle usually is about 85 degrees, and 
the hinge-area is relatively lower. In the former, the three ridges 
formed by the posterior slopes of the cardinal process and the 
proximal terminations of the two crural plates, as seen through 
the delthyrium of the complete shell, diverge less. The median 
parts of the brachial valve are traversed by two diverging ridges, 
thickening anteriorly, and sometimes separated anteriorly by 
two much lower vascular ridges. On each side of the prominent 
pair of median ridges is an adductor scar. A low ridge, indistinct 
posteriorly, divides each scar into two parts longitudinally. In 
the specimens from the Liberty and Waynesville beds, the exterior 
lateral outline of the adductor areas is approximately crescentic 
in form, and is fairly well defined posteriorly; anteriorly the exte- 
rior half of the adductor scar converges toward the longitudinal 
ridge separating it from the inner half. In the specimens from 
the Eden beds, the anterior outline of the adductor scars tends 
to be more quadratic, the anterior termination of the exterior 
half of each scar being broader, and more in line with the anterior 
termination of the inner half of the same scar; posteriorly, the 
exterior lateral outline of the adductor scars frequently is rather 
indistinct. 
