1889 .] 
327 
[Foerste. 
which the ratio of length to width is less than the ratio of 8 to 10, 
and hence are confined almost exclusively to dorsal valves. 
The radiating plications are crossed by concentric striae, usu- 
ally not visible in our specimens except on top of the plications 
where they become elevated folds or even have the aspect of tuber- 
cles. These tubercles may be fairly distinct on the anterior half 
of the shell. The plications are all unbranched. 
The average size of specimens would be perhaps a length of 7 
mm., and a width of 8.1 mm. Specimens, are, however, frequently 
much larger and may be 8.5 mm. long, 10.2 mm. wide. 
Fenestella. 
A single specimen was found at Cumberland Gap, Tennessee, 
which is readily referred to this genus but its specific identifica- 
tion is more difficult. Six branches occur in a width of 2 mm. and 
four and one-half, sometimes five dissepiments in the same length. 
The fenestrules are short and narrow. No pores can be seen. The 
dissepiments are rather thick. 
Ptilodictya expansa, Hall and Whitfield? 
The single fragment found at Cumberland Gap, Tennessee, is 23 
mm. long and 10 mm. broad. From nine to ten rows of cells are 
found in a width of 2 mm., and from six to six and a half cells in a 
length of also 2 mm. As well as may be determined from the spec- 
imen the cells are four sided, rhomboid, arranged in longitudinal 
rows and also in horizontal rows which run from a few median rows 
of cells to the lateral edges of the frond, forming angles of 15° with 
the median rows. The state of preservation of the specimen was 
not sufficient to determine whether these median rows were differ- 
entiated in size and form from the lateral rows, as is usual in this 
species ; nor could it be determined whether the lateral edges were 
strictly parallel, but it is not uncommon to find the same species at 
Dayton, Ohio, with the lateral edges fairly irregular, at least not 
parallel. It is possible that the forms with irregular edges should 
be separated from those in which the edges of the fronds are par- 
allel as a distinct species. This idea is strengthened by the fact 
that in the Ohio specimens the forms with parallel edges have flat 
fronds and in my own experience have been found only at Todd’s 
Fork, while those with irregular edges have a tendency towards 
undulate fronds, especially in the larger individuals, and are of 
much wider distribution. 
