336 
Aug. F. Foerste 
about three-fourths of a millimeter, to a point 15 millimeters behind 
the beak, where the thickness equals about half a millimeter, 
excepting along the immediate vicinity of the hinge-line. Toward 
the basal margin, the thickness of the valve diminishes to scarcely 
more than a sixth or a seventh of a millimeter. 
The shell is distinctly wrinkled concentrically, the wrinkles 
being quite strong anteriorly, from the umbonal area as far down as 
the basal margin, although not quite as distinct as in the umbonal 
region of Sedgwickia divaricata Hall and Whitfield. 
Transverse, low, rounded plications, distinctly defined anterior- 
ly, where the shell is preserved; less distinctly defined posteriorly, 
along the hinge-line, where the shell is partly exfoliated; almost 
invisible along the lower, posterior part of the internal cast of the 
right valve, where all of the shell substance is gone. 
Faint, broad grooves radiate from the posterior side of the beaks 
along the post-umbonal slope, but are thought to have no special 
significance. Similar grooves are seen occasionally in Modiolopsis. 
Additional information regarding the shell substance of Phola- 
dormorpha pholadiformis is presented by a specimen collected by 
the writer in a limestone layer at the top of the Waynesville member 
of the Richmond, along Cowan creek, southeast of Clarksville, Ohio. 
The outer black coat of the shell is very thin, as thin as paper. 
This is the part which is preserved in the form of a thin black film in 
the fine-grained sandstones of the so-called Lorraine of New York, 
Canada, and Wisconsin. The remainder of the shell is much 
thicker, especially toward the umbonal region, where it reaches 
a thickness of about two-thirds of a millimeter. This part of the 
shell consists of vertical fibers, possible of aragonite, and this is the 
part which usually is not preserved, especially in argillaceous and 
arenaceous strata. Anteriorly, the concentric wrinkles are well 
marked. Posterior to the beak, both along the base and along the 
hinge-line, the transverse plications are well defined, extending 
toward the crest of the umbonal ridge. 
43. Pholadomorpha corrugata, Miller and Faber 
{Plate V, Fig 5) 
1892. Modiolopsis corrugata Miller and Faber, Jour. Cincinnati Soc. Nat. Hist., 
15, p. 79, Plate 1, Fig. 1 
The type of Modiolopsis corrugata forms No. 8813 in the Faber 
collection at Chicago University. It was described as coming from 
