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

1970
controlled by the nature of the underlying
object, and about equal numbers of specimens
represent concave, flat, and convex external
profiles. The umbonal region of the left valve
varies from slightly convex to moderately convex,
and the beak area rises only slightly above
the cardinal area in a hinge view. The principal
growth gradient of the left valve umbonal area
is commonly downward (infracrescent), rarely
forward (procrescent). Commonly, the beak is
situated medially along the hinge, and in numerous
specimens it is slightly anterior of the midpoint.
The subequal auricles are set apart from the
main body of the valve by flattening and by
subauricular sinuses of the valve margin; these
indentations are essentially equal in a single
valve. The vertical profile beyond the attachment
area tends to be slightly convex to flat in
both left and right valves. The curvature of the
NEWELL AND BOYD: PERMIAN BIVALVIA
surface as viewed along the plane of commissure
is constant, but some individuals exhibit a
steeper posterior or anterior slope. Although the
main body of the valve is approximately equilateral,
some specimens are slightly extended
forward or backward.
Early growth of the right valve produced an
infracrescent, unattached pectiniform stage
(fig. 25H, I). This stage persisted to a height of
2 to 4.5 mm., beyond which the valve became
cemented to the substrate (fig. 26C). The few
specimens with a well-preserved umbonal area
show a byssal notch in the pectiniform stage.
The trace of the notch, delineated by growth
lines, varies from 1 to 2 mm. long. In most specimens
the pectiniform stage is not sharply differentiated
from the adjacent part of the valve. In
one specimen the hinge line of the pectiniform
stage is along the edge of the triangular cardinal
area and anterior to the apex.
FIG. 26. Pegmavalvula gloveri Newell and Boyd, new species, Permian, Road Canyon Formation, Glass
Mountains, western Texas. A, B. Left valve, U.S.N.M. No. 155006; U.S.N.M. 721j. C, D. Right valve,
U.S.N.M. No. 154987, cemented at umbo to shell fragment; U.S.N.M. 721s. E. Shell fragment, U.S.N.M.
No. 154988, showing cylindrical spines; U.S.N.M. 721j. F, G. Holotype, U.S.N.M. No. 154989, left valve;
U.S.N.M. 726d. A-D, F, G, x 1; E, x 2.
265