GASTEROPODA OF THE LOWER GREEN MARLS. 47 
PERISSOLAX DUBIA. 
Plate m1, Figs. 9-11. 
Purpuroidea? dubia Gabb: Proc. Acad. Nat. Sci., Phila., 1860, p. 94, Pl. m1, Fig. 
11; Synopsis, p. 73; Meek, Check List Cret. and Jur. Foss., p. 21; Geol. 
N. J., Newark, 1868, p. 730. 
Shell of medium size, conical above, abruptly contracted below the 
largest part of the last volution, and extended in front into a short, some- 
what slender beak; volutions about four, strong, convex on the surface, with 
well-marked sutures; apical angle in the vicinity of fifty degrees; aperture 
ovate, somewhat acute at each end, but prolonged below; surface marked 
by revolving ridges and by closely arranged vertical folds; of the former, 
there are eleven, eight of which may be said to be above the middle of 
the volution, or above the periphery, while three only are really below this 
point, and these more distant and somewhat stronger than the others, with 
distinctly concave spaces between, while the lower half of the space between 
the beak and the periphery seems to be destitute of ridges; vertical 
folds low and rounded, with concave interspaces of about an equal breadth 
with the folds, or the folds may be said to unite at their bases, occupying 
the entire space; twelve of them can be counted on the outer half of the 
last volution; the folds bent slightly backwards from the suture to the cen- 
ter of the volution, and again very faintly forward at that point, below 
which they rapidly become obsolete, not showing on the under side of the 
volution. | 
The specimen from which the above description is taken is entirely a 
cast, in very soft, friable marl, and is rapidly falling to pieces, notwith- 
standing every effort has been made to preserve it, the decomposition hav- 
ing progressed too far before it came into my hands. It appears to bea 
_east retaining the external features, the yielding marl having been pressed 
into the cavity as the shell substance was removed; it thus retains the 
external features and presents the true characters of the shell. It is much 
larger than Mr. Gabb’s type specimen, which is also a cast, but partially if 
not wholly an internal one, so that the surface features were not fully pre- 
served. There can be but little if any doubt, however, of their specific 
dentity. I am not sure that the present specimen retains the entire length 
