Strophomena and Other Fossils 
75 
division to the base of the lower Hehertella insculpta horizon. 
The specimens are quadrate in outline^ and the radiating striae 
are subequal in size, or alternately larger and smaller. As a 
matter of fact, the variety Strophomena elongata is more common 
in the Clarksville bed, at this locality, than the more typical 
Strophomena planumbona, which has a shorter hinge-line. It is 
evident that Strophomena planumbona is connected with Stro- 
phomena elongata by specimens in which the postero-lateral 
angles are only moderately acute. Specimens of this type become 
common in the upper part of the Blanchester division and in the 
lower half of the Liberty. 
No Strophomenas occur in the lower part of the Blanchester 
division. Specimens of Strophomena planumbona, with a quad- 
rate outline, come in at the same horizon as the first specimens 
of Strophomena nutans and Strophomena neglecta. Near the 
middle of the Blanchester division, some of these quadrate forms 
have a rather high hinge-area, considering the size of the shell, 
with a corresponding increase of the size of the cavity enclosed 
between the two valves. Moreover, some of the specimens have 
the alternation of three finer striae with single larger striations, 
which is characteristic of the type of Strophomena planumbona. 
In fact, it is probable that the types of Strophomena planumbona 
were secured from the upper half of the Blanchester division of 
the Waynesville member. In some of these specimens, the alter- 
nation of three finer striae with a single stronger striation does 
not continue as far as the anterior margin of the shell, the striae 
on the deflected anterior slopes being more nearly subequal. 
Specimens of Strophomena planumbona, with a quadrate out- 
line and with subequal radiating striae, occur also in the Liberty. 
Here they are associated, however, with more numerous speci- 
mens in which the postero-lateral angles are more acute, and in 
which the oblique wrinkling, characteristic of the type of Stro- 
phomena subtenta, is seen more frequently. In fact, it is probable 
that the type of Strophomena subtenta was obtained from the 
lower part of the Liberty. Similar specimens occur in the upper 
part of the Blanchester division, but in these the oblique wrink- 
ling is much rarer. As a matter of fact, the oblique wrinkling 
occurs occasionally even in the quadrate forms of Strophomena 
planumbona found in the upper part of the Blanchester division. 
