56 
Aug. F. Foerste 
11 feet thick, are identified as Strophomena maysvillensis, while 
the specimens in the overlying part of the Fairmount, 47 feet thick 
according to measurements taken along a long gradient, are more 
like the typical specimens of Strophomena planoconvexa 
Somewhat similar phenomena are noted in Madison county, 
Kentucky. Here the specimens of Strophomena which occur in 
the upper part of the Fairmount, beneath the Tate layer, more 
closely resemble Strophomena planoconvexa, while those in the 
lower half of the Fairmount, and in the Mount Hope bed, are 
identified as Strophomena maysvillensis. These features are well 
presented by the section along the railroad, several miles west 
of Richmond, and also by the road leading west from the Rich- 
mond pike, 1 mile north of Paint Lick. 
Strophomena occurs in quite considerable numbers, associated 
with Orthorhynchula linneyi, along the Cumberland River, from 
the northwestern edge of Wayne to the northeastern part of Cum- 
berland county. Some of the specimens have the flat, more quad- 
rate form characteristic of the type specimens of Strophomena 
planoconvexa. Others have the more triangular form, character- 
istic of Strophomena maysvillensis. 
At the time when the specific name Strophomena maysvillensis 
was proposed, it seemed possible to distinguish two species, one 
of which was smaller, flatter, and more quadrate in outline, while 
the other was a larger and more triangular form, more strongly 
arched across the middle. The fact that in some parts of the field, 
the flatter, more quadrate form occurred at a higher horizon, rather 
favored this view, even if this separation by horizons could not 
be maintained when tracing these fossils to other areas. However, 
more recently, the conviction has grown that the larger, more 
triangular form is likely to be present wherever conditions strongly 
favored the development of the species, so that the specimens grew 
larger, developed more vigorous shells, and became very numerous. 
On the contrary, where conditions were less favorable, the speci- 
mens were smaller, the valves were thinner, and flatter, and the 
outline approached more nearly the ordinary quadrate form. 
From this point of view, Strophomena maysvillensis may be 
regarded as simply a more healthy form of Strophomena planocon- 
vexa, the type specimens of the latter having been collected so far 
north of the area in which the growth of the species was most 
vigorous, that only the more depauperate specimens were found. 
