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

1970 NEWELL AND BOYD: PERMIAN BIVALVIA
263
in other Leonardian collections but is rare in
younger material.
A third grade of surface is recognized in
Form C (figs. 19, 20). These shells are nearly
devoid of ornamentation over the mature shell
except that localized areas of some valves show
a tendency for the development of obscure,
coarse radial ribs, and fine costellae over juvenile
area of umbones. This type of ornamentation
predominates in a small area in the Road Canyon
Formation of the Glass Mountains, western
Texas (U.S.N.M. 703, 703c), but it also occurs
sparsely at other horizons.
The new species is based on a fair number of
silicified specimens, all from the Glass Mountains,
western Texas. Form A is represented by
19 specimens that adequately display surface
ornamentation and marginal profiles, plus 54 in
which inner surface features are shown.
Form B is represented by five specimens with
intact margin, and 32 more fragmentary valves.
Internal characteristics are preserved in material
from five localities.
Form C is represented by one nearly complete
valve and 20 instructive fragmentary specimens.
Four localities yielded specimens showing internal
details.
In addition, there are hundreds of shell fragments
that provide useful information about
ornamentation, but little else.
PEGMAVALVULA NEWELL AND BOYD,
NEW GENUS
Figures 7A, B, H, I, 25-27
TYPE SPECIES: Pegmavalvula gloveri Newell and
Boyd, new species.
DIAGNOSIS: Small pectiniform, cemented,
nearly equivalved pseudomonotids with relatively
uniform radial ribs and small auricles;
byssal notch lost early in growth.
DISTRIBUTION: Leonardian and early Guadalupian,
western Texas.
DISCUSSION: Some individuals among these
shells are characterized by moderate procrescentic
growth (figs. 25A, B, 27A), which, in
context with a fairly well-developed posterior
auricle and valves of nearly equal convexity,
present a general expression quite similar to that
of the aviculopectinid genus Streblochondria
(Newell, "1937" [1938]). Consequently, we
considered the possibility that Pegmavalvula is a
cemented version of one of the Streblochondriinae,
such as Guizhoupecten Chen. This hypo-
thesis envisioned an evolutionary parallelism
with the ancestral-descendent relationship seen
in the free scallop Chlamys and its cemented
derivative, Hinnites (fig. 3). The relationship of
Pegmavalvula however, more probably is with
Prospondylus, which it resembles more closely
than any member of the Streblochondriinae.
Pegmavalvula is distinguishable from other genera
of the Pseudomonotidae by being nearly, but not
quite, equivalved and equilateral; from the
Streblochondriinae it is distinguishable by its
cementation and spinosity.
Pegmavalvula gloveril Newell and Boyd, new
species
Figures 7A, B, 25, 26
DIAGNOSIS: Nearly equilateral, short-hinged
and pear-shaped, with subquadrate auricles and
broad subauricular sinuses; ornamentation consisting
of weak plications corresponding to
coarse costae bearing tangential cylindrical
spines; these separated by radial sulci bearing
several fine costellae; origination of successive
ranks of costae on left valves generally by intercalation,
on right valves by indistinct bifurcation;
surface elements commonly appreciably
coarser and more sharply defined on left than
on right valves.
DISTRIBUTION: Leonardian-early Guadalupian,
western Texas. Bone Spring Limestone
(Leonardian): U.S.N.M. 728f. Road Canyon:
U.S.N.M. 721j, 721s, 721z, 726d. Willis Ranch:
U.S.N.M. 706, 706e. Lens between Willis Ranch
and Apple Ranch: U.S.N.M. 706b. Lower Getaway:
A.M.N.H. 512, 600; U.S.N.M. 728, 730.
DISCUSSION: Two slightly different forms are
recognized. A long-ranging and widely distributed
form, Form A (fig. 25), is characterized
by recurved spines (fig. 25G), incipient plication,
or heavy ribs separated by broad radial
furrows bearing from four to nine fine costellae
(fig. 25A). Form B (fig. 26), restricted in our
collections to the Road Canyon Formation, has
somewhat different ornamentation. The ornamentation
consists mainly of irregular radial
rows of imbricating tangential spines reflected
internally as plicae. The external furrows be-
1 Named in honor of Mr. and Mrs. Walter H. Glover,
landlords of the Pine Spring Ranch near Guadalupe Pass,
at the foot of the Guadalupe Mounitains, western Texas.
These fine people have been benefactors and hosts to
several generations of students of the Permian rocks of that
region.