198 
PALAEONTOLOGY OF NEW YORK. 
which is moderately convex; the ventral margin (or that corresponding 
to what may be termed the ventral side of the shell) is regularly curving. 
The umbo is situated about three-fourths of the width from the base, and 
extending thence on each side, almost rectangularly to the transverse 
axis, is a distinct fold which gradually expands towards the margins, and 
below which is a corresponding groove or channel. Above this fold is a 
narrow border or flange, which turns upward at a considerable angle 
(varying in two specimens from ten to more than thirty degrees), and in 
the centre of which is an abrupt angular depression (see figs. 27 and 28 
of pi. 32). Shell thin, and usually very imperfectly preserved. 
Surface of the shell concentrically or transversely striated, the marks often 
appearing in the casts. In well-preserved specimens these striae are 
cancellated by extremely fine longitudinal ones, which are visible only 
under a strong lens, and are not preserved in all specimens. These striae 
are much more conspicuous near the base of the shell, and especially on 
the ventral margin of the aperture. Surface of the operculum distinctly 
striated concentrically, and also obscurely marked by radiating striae; the 
margin of the flange is striato-plicate. 
In addition to the ordinary surface striae, the dorsal face sometimes presents 
a distinct longitudinal groove on each side, at about one-third the distance 
between the lateral margin and the angular centre. These lateral grooves 
are shown in figs. 23 and 28, drawn from two different individuals, the latter 
being an enlargement. On the flattened ventral surface there are often two 
lines of depression or fracture, essentially opposite to the distinct dorsal grooves. 
This feature is slightly marked in fig. 22 (the ventral side of fig. 23), and 
more distinctly in fig. 30, which is seen from the inner side of the shell. 
Similar lines of fracture are visible in nearly all the compressed specimens of 
this species. See also plate 32 A, fig. 24. 
In a few specimens the surface of the shell near the apex is marked by 
transverse undulations, and in one specimen with sharp transverse lines. These 
are the only indications of septa thus far observed, and are by no means satis¬ 
factory evidence of such structure. 
