Stropliomena and Other Fossils 
125 
means of transmitted light, although not as conspicuous as in the 
case of the pedicel valve. The solution of the problem is seen 
on examining the interior of the valves. The interior of the 
brachial valve is slightly thickened along the hinge-line, this 
thickening being narrowly linear, but increasing slightly in width 
toward its junction with the thickening bordering the lateral 
and anterior parts of the valve. In many specimens this interior 
thickening along the hinge-line is crossed by a series of minute 
transverse ridges, which, however, do not intrude upon the hinge- 
area. In the case of the pedicel valve, a similar series of minute 
transverse ridges is seen upon the linear callosity filling in the 
space between the hinge-area and the immediately underlying 
part of the shell. The shell material composing these transverse 
ridges is deposited by the mantle in an undulating manner, the 
transverse ridges forming the rising parts of the undulations, and 
the supposed wrinkles, seen when the shell is viewed by means of 
transmitted light, correspond to the depressions of the undula- 
tions. The phenomenon evidently is one dependent upon the 
laws of refraction of light, and is connected not with any actual 
wrinkling of the hinge-area or of the exterior surface, but with 
the undulating surface of deposition of the shell material along 
the linear callosities lining the interior of the valves immediately 
within the hinge-area. From these statements it will be noticed 
that the supposed wrinkles of Pleciambonites do not originate 
along the line of junction of the hinge-area with the posterior 
line of the exterior surface of the shell, and hence can not have 
led up to the hollow spines along the posterior margin of Chonetes. 
Specimens showing these supposed wrinkles are more common in 
the Waynesville bed than in the Eden beds, so that this feature 
can not be used to distinguish between the two horizons. More- 
over, numerous specimens do not show the wrinkling. 
Among the specimens referred to Plectambonites rugosa, with 
the oblique wrinkles along the hinge-line, many are 12 mm. long 
and 25 mm. wide. It is specimens of this large size which occurred 
150 feet above the Ohio River at Cincinnati, in the Southgate 
member of the Eden, and which formed the types of Plectambonites 
rugosa. However, specimens of large size occur also at the base 
of the Eden. 
In the Waynesville member of the Richmond, Plectambonites 
is common in the lower part of the middle or Clarksville division. 
