42 MALONE JURASSIC FORMATION OF TEXAS. [bull. 266. ] 
ured by Castillo and Aguilera. Three, of which one closely resembles I 
the example of U E. subnodosa Minister," figured in Goldfuss's Petra-l 
facta germanice (PL LXXXVI, fig. 8), are from the foothills west 
of Malone Mountain, about 2 miles north of the southern end of the \ 
mountain, there associated with Gryphcea mexicana, Pinna quadri- 
frons, Pleuromya inconstans, etc. The three others, representing a 
variety of more broadly triangular outline, are from the west base of 
the mountain, only a short distance north of its southern end. The 
latter station, according to Doctor Stanton's notes, may or may not 
represent the same horizon as the former and " is certainly several 
hundred feet above a horizon of Gryphcea mexicanq and Nerinea." 
The specimen, above compared to E. subnodosa, has a convexity of 
about 40 mm. and a lateral profile whose greatest extent, from beak to 
anterior part of base and extent transverse to the same, are respec- 
tively 86 and 55 nun. Corresponding dimensions in one of the larger 
and broader specimens from the second station above specified are 
respectively and approximately oil. 93, and 73 mm. 
The following is a translation of Castillo and Aguilera 's descrip-I 
tion of Exogyra potosina: 
Shell elongated, oblique, laterally curved, of semilunar contour. Might « 
valve inflated, its major convexity situated near the umbo, incurved like the 
shells of Gryphsea ; provided with an obtuse crest that traverses the shell in 
its entire length forming a curve concentric with the anterior margin; beak 
little free and gently recurved laterally. Left valve operculiform. depressed, 
and adapted to the concavity which the right valve forms; beak rudimentary. 
Surface provided with growth-laminae more marked in the left valve, which is 
entirely lamellar; on the right valve are perceived two pliciform nodules, 
obtuse and imperfect, upon the indistinct crest of the shell. Length, 57 nun. 
Castillo and Aguilera further describe the shell as intermediate 
between Exogyra and Gryphaaa, noting that it approaches the latter 
genus both in the general form of the two valves and by preponder- 
ance of the vertical over the lateral curvature of the umbonal region. 
In the Texas specimens, however, at least sometimes, the lateral curv- 
ature seems to preponderate. 
It is possible that the narrower and broader forms represent two 
species; but this does not seem probable, and they are here provision- 
ally regarded as mere phases of one. 
Exogyra potosina in the Sierra de Catorce, has been recorded only 
from relatively high beds, which Castillo and Aguilera have con- 
sidered as Cretaceous. It apparently occurs only high in the Malone 
district also; but some of its associates in the latter district arc 1 
apparently identical with the lower occurring Pleuromya inconstant 
(of the Theta and Alamitos) ; Gryphcea mexicana (of the Theta and 
Jurassic of Oaxaca) ; Pecten insutus, Pinna quadrifrons, Lucinq 
" Left, of authors generally, who regard the operculiform valve as the right in the 
inequivalve forms of this genus. 
