84 MALONE JURASSIC FORMATION OF TEXAS. [bull. 2| 
trigonal. Buccal region little developed, extremely small compared with t 
anal region; excavated below the beaks, with its extremity rounded in some im 
viduals, in others obliquely truncated, and salient in its lower part, which 
continuous with the ventral border forming an obtuse angle. Beaks situat 
in the anterior third of the shell, varying their position in the different in! 
viduals from nearly terminal to subcentral ; broad, convex, sometimes prosei 
ing two obtuse keels, one on each side of the apex of the beak. Anal region ve 
large, convex, or depressed, narrowed toward its extremity, which is round* 
Dorsal border straight or slightly concave on the anal side, considerably ex( 
vated on the buccal side. Ventral border nearly straight, feebly raised at t 
extremities, but notably more at the anal side. Flanks sometimes very convc 
giving to the shell a cylindrical form, and sometimes depressed so that th 
make it cuneiform: the greater convexity is always in the anterior part 
the shell and below the beaks. Surface ornamented with concentric furro 1 
which contrast with the line lines of growth. Length, .'54 to 63 mm.; bread 
with relation to the length, 0.47 to 0.04; thickness with relation to the lengi 
0.3] to 0.43. 
Castillo and Aguilera supplement this description with the follow 
ing remarks : 
We possess of this species only molds and imperfect examples, but these sIk 
the ornamentation of the shell and its very feeble thickness. The variation 
the characters of this species is not limited merely to the contour and the oth 
details of form of the shell, but is also manifested in the mode of occurrence 
the shell, which is oquivalve in the more convex forms and evidently becon 
more and more inequivalvc in so far as it reaches forms in which the rig 
valve is more elevated than the left in such a manner that the dorsal bonl 
before and behind the beaks passes over the dorsal border of the oppos 
valve, exactly ;is in the Gresslyas, to which genus these forms could 
referred without hesitation if they were found isolated, as to some extent t 
umbonal crests of which we have spoken above would make this position of t 
shell in the genus Gresslya more proper, since the obtuse furrow which is foil 
along the umbonal crests somewhat resembles the oblique furrow which t 
molds of the (Jrosslyas present in the right valve and which in these cor 
spends to the internal lamina. Fortunately we have met with abundant exa 
pies which have permitted us to follow all the variations of the shell, formi 
a series whose extremes, taken without the intermediate forms, could witho 
hesitation he referred to the genera Pleuromya and Gresslya, respectively. 
Pleuromya enconstans var. curta var. n. 
PI. XVI 1 1, fig. 4: IM. XIX, tigs. 1. 2. 
Cnneate-oblong to cuneate-ovate, in itch shorter than the lengths 
Pleuromya inconstans that prevail in the Theta member of tl 
Malone formation, and rather compressed; the narrowly gaping p< 
terior region of the shell slightly recurved, obliquely truncate; ant 
rior region closed; ventral margin gently and evenly convex: (\<n'r- 
margin much less excavated before the beaks than that of Pleurom\ 
peregrina d'Orbigny; beaks more nearly terminal than in that specie 
often unequally elevated, the right being the higher, as seen in son 
