342 The American Geologist. J"""^' ^9^3. 
They were found at Blanchester, Ohio, in the Middle Rich- 
mond. The wrinkhng of the shell along the hinge-line is 
scarcely a specific character, and may be absent. Strophoiiiena 
vetusta makes its first appearance in the lower part of the 
^Middle Richmond and is usually abundant in the middle and 
upper part. 
Sfroplionicua approximata is a subtrigonal form of Str. 
vetusta. The type specimens were found in Dearborn county, 
Indiana, in the Middle Richmond. 
All of the exposures in the immediate vicinity of Richmond, 
Indiana, belong to the Middle Richmond. The base of the Mid- 
dle Richmond is not seen. Strophomena suhtenta occurs only 
at the very base of the exposures, while Strophomena vetusta 
occurs at all levels. At the top of the exposures Stropliomcna 
sulcata is rather frequent. 
The base of the Upper Richmond. 
Along the northern line of outcrop, in Ohio and Indiana, 
the richly fossiliferous clays and limestones of the Middle 
Richmond are followed by a considerable thickness of clays in 
which fossils are much less abundant or even absent. These 
upper clays are referred to the Upper Richmond. Farther 
south, in the vicinity of Laurel, Indiana, the equivalent beds 
consist of a nearly unfossiliferous clay rock, forming steep 
walls at the base of several waterfalls but disintegrating read- 
ily under the influence of weathering. At Madison, the equiv- 
alent beds consist of an earthy, siliceous limestone which is 
harder than the clay rock at Laurel and is well bedded so that 
at one time it was quarried. Its appearance, however, has 
proved to be deceptive, and this rock is now known to be 
strongly affected by weathering. The main body of this rock 
at Madison does not contain fossils and elsewhere it usually 
contains so few fossils that its geological position long re- 
mained unknown. L^pon lithological grounds it has been re- 
ferred in former times to the Silurian, but an unmistakable Or- 
dovician fauna is now known from the body of this usually 
nearl}- unfossiliferous section, and an abundant Ordovician 
fauna overlies it at Madison. 
The separation of the Upper Richmond from the Middle 
Richmond must usually be effected by lithological means. 
The ^Middle Richmond consists of a series of richly fos- 
