76 
Aug, F. Foerste 
Specimens with a successive alternation of three finer radiat- 
ing striae with a single stronger striation occur also among the 
forms with moderately acute postero-lateral angles, which are • 
found in the lower Liberty. 
It is possible that Strophomena planumbona ranges up into the 
Whitewater bed. There is no doubt that Strophomena vetusta 
is the predominating species in the Whitewater section, and occurs 
here almost to the exclusion of other forms, but occasionally 
specimens are found which resemble Strophomena planumbona 
so much that the presence of the latter species in the Whitewater 
member can not be denied without further investigation. 
Strophomena planwnbona and Strophoinena subtenta were de- 
scribed by Hall in vol. I, of the New York Paleontology. Since 
Strophomena planumbona was described on an earlier page, this 
name should be adopted for both forms if Strophomena subtenta 
proves not to be distinct. 
The types of these forms are readily distinguishable. In 
Strophomena planumbona, there is a repeated alternation of three 
finer striae with a single more prominent one. In the type of 
Strophomena subtenta, on the contrary, the radiating striae are 
more nearly subequal. The type of Strophoinena subtenta, more- 
over, is obliquely wrinkled along the hinge-line. 
An attempt to distinguish these forms in the field, however, 
where the specimens are very numerous, fails to substantiate 
the distinctness between these two supposed species. It is 
evident, for instance, that the oblique wrinkling, in Strophomena 
subtenta, is conspicuous only in selected specimens, and that it is 
entirely absent in the majority of specimens which must be referred 
to Strophomena subtenta, since they occur in the same layers and 
have the same exterior and interior markings, excepting only the 
presence of the oblique wrinkling along the hinge-line. More- 
over, it is not possible to distinguish between Strophomena plan- 
umbona and Strophomena subtenta on the basis of the exterior 
markings. In the great majority of specimens, the radiating 
striae are subequal in size, or are alternately larger and smaller. 
Specimens in which three finer striae alternate repeatedly with a 
single stronger striation are rather few in number and occur 
associated with the former in such a manner that it is very evi- 
dent that they represent only individuals belonging to the same 
species, and not even a distinct variety. 
