Notes on Cincinnatian Fossil Types 
329 
termediate points, Byssonychia robusta occurs associated with 
Dystactospongia madisonensis at the base of the massive Tetradium 
layer at the base of the Saluda member of the Richmond group. 
The original of the type represented by figure 3, accompanying 
the original description, is used for figures lA, IB in the present 
bulletin. The original of figure 3a accompanying the original 
description is represented in this bulletin by figure 1C. The latter 
specimen is unquestionably a typical example of the species describ- 
ed later by Ulrich as Byssonychia richmondensis, and is evidently 
the type which Miller had in mind when he described the species 
as having its ''anterior side flattened and depressed in the region 
of the byssus; beaks acute, triangular,” and refers to the "abrupt 
bending over of the shell on its anterior side.” 
Formerly I thought that the original of the figure 3, presented 
by Miller, might represent a relatively broader species, distinct 
from Byssonychia richmondensis. Later, however, I had the op- 
portunity of seeing hundreds of specimens of Byssonychia robusta 
at its type horizon, where it often is very abundant, and it soon 
became evident that the broader specimens, as represented by the 
specimen used for figure 3, by Miller, were the normal forms, while 
the more elongate forms, suggesting Byssonychia richmondensis, 
as figured by Ulrich, showed indications of compression antero- 
posteriorly. This compression not only gave a more elongate 
appearance to the shell, but made the angular bending of the shell 
along the umbonal ridge more abrupt, and made both the plications 
and the interspaces between these plications more narrow, although 
of course, not changing their number. From this it must not be 
assumed that the broader form, used by Miller for figure 3, does 
not show any abrupt bending over of the shell anteriorly. This 
abrupt bending is confined usually to the upper half of the shell. 
Along the lower half, the umbonal ridge is merely strongly rounded. 
The very angular, almost acutely angular bending of the shell is 
confined to those parts within 20 mm. of the beak, and from this 
distance downward the angularity of the umbonal ridge gives way 
gradually to more and more pronounced rounding. In this respect 
the original of figure 3, as published by Miller, was as typical as 
the original of his figure 3a, but in the former specimen the shell is 
not preserved along the angle of the umbonal ridge near the beak, 
but comparison with numerous other specimens from which the shell 
substance has been removed indicates that the cast of the interior 
