On a New Exogyra From the Del Rio Clay and Some Observations on the Evolution of Exogyra in the Texas Cretaceous Page: 5

                                
On a New Exogyra from Del Rio Clay

line to immature specimens of Ex. texana, with 5 to 8 radial
folds on the anterior slope, a distinct umbonal ridge, and a
steep anterior slope. The upper valve is practically smooth.
This form seems to be intimately related to Exogyra Bous-
singaulti d' Orb.' which occurs approximately in the cor-
responding horizon of Europe. A similar form has been
found in the Aptian of San Juan Raya, Puebla, Mexico.
This species seems to have branched off into several forms
in the next higher division of the Trinity formation, some
of which have been described and very imperfectly figured
by Cragin as Ex. paupercula2 and Ex. weatherfordensis.
These species are quite frequent in the Alternating beds
(Glenrose), the middle and upper part of the Trinity form-
ation. Both seem to be related to Ex. Hilli and Ex. Bous-
singaulti, although the distinguishing feature in Ex. pau-
percula is that the beak becomes free from the rest of the
shell as in Ex. arietina. Ex. paupercula has only 2 to 4
irregularly-topped, angular, radial folds on the lower valve.
while the upper valve seems to be smooth. I have not been
able to see the original specimens.
Much more similar to the Ex. Boussingaulti group is the
Ex. weatherfordensis (pl. 1, fig. 1, 2) with a strong um-
bonal ridge, 6 to 12 oblique plications on the anterior slope,
and a greater number of narrow, subradiate, or curved pli-
cations on the posterior slope. The upper valve is smooth,
but the margin is often scalloped corresponding to the end-
ing of the ribs on the lower valve.
This is, at least, what I take to be Ex. weatherfordensis,
a species which has not been very well figured. In the
collection of the University of Texas exists a number of
specimens collected by Taff at one quarter of a mile west of
Weatherford, labeled E. Boussingaulti d'Orb. This species
certainly is very similar to the French form and a near rela-
tive of it, and at the same time also of Ex. Hilli. It does
1A.d' Orbigny, Voy. dans 1'Amdrique merid. t. 111, 4me part, Pal.,
p. 91, pl. 18, fig. 20; pl. 20, fig. 8, 9.
Idem, Paldont. franc., Terr. Crdt. t. 111, p. 702, pl. 468.
2Cragin, loc. cit., p. 186, pl. 30, fig. 7, 8.
3Idem, ibidem, p. 188, pl. 45, fig. 7-10.