50 MALONE JURASSIC FORMATION OF TEXAS. [bull. 266. 
coming at first feebly and then, on the middle and posterior region, 
strongly and abruptly obtuse-angulated along a radial line a little 
above the middle, so that either valve presents two slopes — a dorsal, 
which is flat or in part feebly concave, and a somewhat wider ventral, 
which retains some convexity even on the posterior part; dorsal slope 
of either valve ornamented with 10 to 15 compressed and gentlj 
elevated radial costellse, which are separated by intervals wider than 
themselves and obliquely scored by numerous growth lines that give 
the costellse a more or less granulated or crenulated appearance; ven- 
tral slope marked only with strong, pliciform, for the most part very 
oblique growth lines. The summit of the supra-median angle is the 
locus of a linear hingelike plait, or gore, by whose ready erosion the 
valves are often cleft, especially along the middle and posterior 
region, each into two valvelike segments, so that broken specimens 
which include only that region appear not only tetragonal and quadri- 
lateral, but also quadrivalvate. 
Measurements. — Height (across larger cud). 50 nun.; breadth 
(across same), 30 mm. ; length, 240 mm.; all approximate. 
Occurrence. — One and a half miles east of Malone station. This 
is one of the most abundant fossils of the Theta subdivision in that' 
locality and ranges nearly throughout the known fossiliferous thick- 
ness of it. It usually occurs in sections varying in size from less 
than 1 inch to I or 5 inches long, of which about so were here ol 
obtained. Some have been subjected to pressure in such a manner as 
to make them nearly square in cross section. Doctor Stanton also 
obtained several on the west side of Malone Mountain, as follows: 
One in No. lV> of his Malone Mountain section, 2 miles south of west 
Malone station; 2 from a foothill about 2 miles northwest of the 
southern end of the mountain, and 6 from the west base of the 
mountain, a short distance north of its southern end. Part of those 
(cast>) from the last-named locality are more than usually coral 
pressed, rapidly expanded, and somewhat curved, and at first sight 
suggest a distinct species or variety; but others are of intermediate 
form ami some bear shell remnants with seemingly characteristic 
guadrifrons ornamentation, so that, while individual variation may 
have played a part, the exceptional form should probably be attrib- 
uted chiefly, if not wholly, to the distortive influence of the orogenj 
forces that have acted upon the matrix containing them. • 
AROIDiE„ 
Genus ARCA Linnaeus. 
Arca taffi] s]>. n. 
TM. VI. Qgs. 7. 8. 
Shell small, ventricose, broader than high, of moderate length; 
beaks strongly arched, inrolled. and brought into contact; posterior 
