IDY TE1>t' I- - American Museum of Natural History

1970 NEWELL AND BOYD: PERMIAN BIVALVIA
261
anteriorly. Small auricles are present in some
valves in which slight flattening develops laterally
to the umbo. On left valves characterized by
this development, the posterior auricle is the
more prominent.
General growth expansion of both valves proceeded
more or less equally anteroventrally and
posteroventrally, until the shell attained a height
of about 4 cm. From this point, most valves
exhibit accelerated anteroventral expansion.
In mature individuals, the resilifer, which
commonly extends slightly below the rest of the
ventral margin of the cardinal area, is slightly
higher than wide, upright rather than oblique,
and similar in the two valves. Some individuals
display a median fold in the resilifer, interpreted
as a result of pathologic splitting or division of
the resilium (fig. 19F).
Left valves commonly have a large, broadly
rounded umbonal cavity, whereas most right
valves have only a small low recess under the
ventral part of the resilifer. In exceptional right
and left valves the cavity is lacking.
The interiors of both valves possess distinctive
curved buttress ridges, around the visceral
cavity, trending from the posteroventral corner
of the resilifer across the floor of the valve to the
posterodorsal corner of the adductor muscle
scar. In the left valve, this ridge is commonly
less conspicuous, as it coincides with an angular
change in slope from the convex umbonal region
to the flatter marginal zone.
The detailed characteristics of musculature
are not visible. The adductor scar is rather
weakly imprinted on the posterior side of the
valve between one-third and one-half of the
height of a valve below the beak.
The cardinal area is relatively short in both
right and left valves, is two to four times its
height, and is very much shorter than the maximum
shell length. The cardinal area of the right
valve slopes ventrally inward some 110 to 120
degrees with respect to the plane of commissure
(apsacline condition in brachiopod terminology:
Williams and Rowell, 1965, p. H60), and the
highest part of the cardinal area coincides with
the apex of the valve. The cardinal area of the
left valve is approximately parallel to the plane
of commissure or slightly incurved (apsacline).
The umbo rises well above the cardinal area.
The ornamentation over the mature parts of
the shell is similar on the two valves. It is highly
variable, and there is a tendency for certain
combinations of form and ornamentation to
characterize samples from individual localities,
tempting us to conclude that we might be dealing
with geographic races or ecophenotypes. The
tantalizing suggestion of segregation, however,
cannot be quantified or documented with the
small and generally fragmentary samples available.
Three arbitrary classes of ornamentation
in what appears to be a continuum distinguish
Forms A-C.
Some or all of the individuals at all localities
exhibit costae, and these commonly are discontinuous.
This rough and irregular ornamentation
first appears on left valves at a height of 1 to
4 cm., and the fine costellae of the umbonal area
do not continue onto the adjacent mature
surface.
No individual ridge of the mature surface
persists very far before bending downward into
the general surface, terminating as a tangential
hollow spine overhanging the subsequent valve
increment, or terminating as an erect spine by
an upturn of the lamina. Some of the last
mentioned even recurve outward and upward
toward the hinge. New costae appear both by
branching and by implantation. Spines commonly
vary in size, shape, and orientation on a
single valve. In fact, individual spines commonly
vary in these respects throughout their length.
The spines are arched projections of a thin outershell
lamina and thus are hollow and open on
the under side. They vary in frequency from
individual to individual, and in lateral and
longitudinal spacing.
The most common expression, Form A (figs.
22, 23), is characterized by irregular, coarse
costae surmounted by scattered, tangential,
hollow spines. Frequently, these originate as low
ridges and are distinct projections only at their
distal ends. Other ridges, instead of terminating
in low spines, plunge and disappear before
reaching the shell margin. These may give the
effect of crude, discontinuous plications. This
ornamentation is relatively uncommon in the
stratigraphically older specimens of P. acinetus,
but predominates near the top of its known
range, in the Word Formation (Willis Ranch
Member).
A second expression in ornamentation is found
in Form B (fig. 24). These shells have a frilled
appearance due to projecting scales. The scales
are curved upward, and their free lateral margins
expand outward and upward instead of