80 MALONE JURASSIC FORMATION OF TEXAS. [bull. 206 
and a supero-posterior larger area constitute the only markings, al 
of the shell except these terminal areas being crossed by radiating 
ribs, of which there are about 20 on either valve. The ribs are nar- 
rowly compressed and rather strongly elevated (cariniform), ofter 
with irregularly crinkled summits, and some of them are pressed 
backward so that the posterior slope is comparatively gentle, whik 
the anterior is steep or overhangs, giving them somewhat the appear- 1 
a nee of imbrications; they are unequally remote and separated by 
broad, flattish-concave intervals; and one of the posterior ribs is 
usually higher and larger than the others, the two or three that arc 
yet posterior to this, and a few of the anterior ribs, being less pro- 
nounced than the others. The anterior part of the base is gently 
convex, becoming straighter and gradually a little rising posterior tc 
a point between the middle and the posterior end, and again appar- 
ently a little descending or prominent opposite the largest one of the 
posterior ribs, the base describing thus a feebly sigmoid curvature. 
The latter feature is not seen in all specimens, and may be due to 
mechanical distortion. 
Measurements. — The two largest specimens average about 50 mm. 
in height. The breadth is apparently about three-fourths of the 
height, and the height about half of the length. 
Oecurrenoe. — Twenty-five specimens are represented from locality 
1^ miles east of Malone station. 
Pholadomya marcoui sp. n. 
PI. XVI, figs. 1, 2. 
Shell of medium or small-medium size in its genus, very inequi- 
lateral, short-oblong or ovate-oblong, more or less rounded at both 
extremities, ventricose; posterior cardinal border slightly excavated; | 
the posterior region convex-cuneately compressed, gaping narrowly at 
the upper posterior border; beaks situated near the anterior end, ele- 
vated about as in Pholadomya Candida Sby., or P. glabra Ag. ; " sur- 
face, except small anterior and posterior dorsal portions, ornamented i 
with a few (about 6?) remote, feebly expressed, or obsolescent, carini- 
form, radial ribs, the posterior one of which is directed nearly straight 
backward (slightly downward), and also unevenly marked with 
coarse concentric growth lines. 
Measurements. — Height, 36 mm.; length, 56 mm.; breadth, 32 mm. 
Occurrence. — A mile and a half east of Malone station. This 
Pholadomya seems to be less common than either P. tosta or P. 
dumbli. Ten examples were obtained. 
This or a closely related form was found with Exogyra potosina. 
"Referring here to the species as presented in pis. 108 and 100 of Tryon's Structure 
and Systematic Conchology. 
