28 
of the cardinal area. Crural plates triangular, bearing upon their outer 
surface the rather shallow, triangular dental sockets. The whole cardinal 
region has been thickened by silicification, which also accentuates the 
striking ornamentation. 
In size and outline Petroria resembles Plectambonites , but the orna- 
mentation and the interior readily distinguish it. The divided cardinal 
process suggests a relationship with Leptaena or Rafinesquina, though 
the form of the process and the crural plate is quite different, and the 
external outline is not at all similar. Its exact affinities cannot be found 
until more and better interiors have been procured. 
Petroria rugosa n. sp. 
Plate V, figures 15, 16, 17, 18 
Shell small, pedicle valve very convex, brachial valve concave, width 
greater than length, averaging 12 mm. or 13 mm. and 5 mm. or 6 mm. 
Greatest width at the hinge-line, the extremities of which are produced 
into rather sharp angles, the anterior margin rounded, making the outline 
approximately semicircular. Cardinal areas flat, almost equal in height, 
that of the pedicle valve slightly higher and inclined away from the brachial 
area at an angle of about 100 degrees, the pedicle valve being longer than 
the brachial by the width of the pedicle cardinal area. Some of the 
development of the very peculiar ornamentation may be due to the silica 
deposits, but in any event the ornamentation must have been unique. 
Except for a small portion in the very early stages, the surface is covered 
with concentric rows of flat, shingle-like plates, the loose ends of which 
turn up at the beginning of the next concentric row. The ends of the last 
row on the anterior margin are at right angles to their length. The shingle- 
like plates besides being in concentric rows are arranged in radiating 
rows along the lines of radiating striae. 
Pedicle valve very convex, showing a more decided development of 
the unique striae, having a small, round perforation at the apex, delthyrium 
narrowly triangular, covered by a convex deltidial plate. Teeth small, 
triangular, just at the ridge of the base of the delthyrium. Interior not 
known. 
Brachial valve concave, foramen triangular, filled by the projection 
of the cardinal process, extending beyond the plane of the cardinal area 
and trespassing on the area of the pedicle valve. Only a part of the 
interior has been uncovered and it is probable that each feature has been 
made heavier by later deposits, but the shell must originally have been 
stoutly built. Septum showing only the top, broad and rounded, thick- 
ening and strengthening the shell. Cardinal process two stout apophyses, 
apparently sessile or nearly so, that broaden and continue the end of the 
septum, projecting through the foramen. From the outside of the shell 
their thick ends resemble two plates, covering the brachial foramen, with 
their longer axis at right angles to the cardinal area and separated from 
one another by a groove. Crural plates represented by a thick, triangular 
projection, the outside surface of each bearing a shallow tooth socket. 
