RAGiN.] DESCRIPTIONS OF SPECIES. 83 
^yschetzku and Astarte malonensis. The material collected by Doctor 
■Stanton includes part of that from the first two of these and also 
specimens from the following localities: Two miles west and also 
bout the same distance south of west of Malone station (here in 
titfos. 13 and 25 of his Malone Mountain section) ; foothill west of 
&( Malone Mountain, 2 miles north of its southern end; and west base 
it )f the mountain, near its southern end. 
On the anterior part of the umbonal region of this shell there 
hlescends to the base a zone of constriction, as if the shell, when 
oinlastic, had been pinched, making the valves there gently concave 
poind the base a little reentrant, a feature which is often pronounced, 
ai hough sometimes scarcely perceptible. When viewed from below, 
he base is usually seen to be twisted, presenting a more or less decided 
iiiight- and -left sigmoid flexure. The right hinge margin overlaps the 
» eft. The shell gapes narrowly at the posterior end. 
This fossil well sustains the reputation of the Pleuromyas for 
friability, meriting its specific name even better, if possible, in 
oilTexas than in Mexico. Manifold shades of form present themselves — 
liejome natural, others dynamical modifications of these — but all seem 
o intergraduate, and the specific place of many of them becomes 
ndeterminable the moment an attempt is made to group them in 
Itaore than one species. For this reason and because some of the speci- 
mens agree perfectly with those described and figured by Castillo 
nd Aguilera from the Upper Jurassic of San Luis Potosi as Pleu- 
omya inconstans, all are referred to that species. 
The cuneate, the subcylindrical. and the Gresslya-like variations 
hat have been recorded from the Sierra de Catorce, all are repre- 
ented in the collections from the Malone district also. What appears 
o be the central phase for the latter district may be described as fol- 
pws: The posterior terminal region of the shell is usually somewhat 
arrowed, sometimes strongly so, and is recurved or obliquely up- 
urned; the anterior margin descends in a convex oblique line to an 
btusely pointed or narrowly rounded corner in which it meets the 
orizontal or often somewhat downward-deflected anterior prolonga- 
ion of the base. The anterior end thus presents a " Roman-nosed,'' 
r " hook-nosed, " appearance, and the shell as a whole, viewed later- 
lly, presents a sigmoid flexure, while the basal margin presents such 
'j> exure whether viewed from the side or from below. This is by far 
( he most abundantly represented form of the species in the Malone 
Histrict, and it attains the largest size. That it also occurs in the 
ierra de Catorce is indicated by Castillo and Aguilera 's very excel - 
pt and comprehensive description of the species, of which a transla- 
ion is here added : 
Shell inequilateral, of variable form, sometimes truncate on the anterior bor- 
er, sometimes with this same border salient; the general dominant form is ovate 
