ORDOVICIAN FAUNAS. 153 
The dimensions of a rather large individual are: length, 18 mm. ; 
breadth, 21 mm.; convexity of both valves, 9 mm. — 
Remarks.—This species is not uncommon in many of the middle 
and upper beds of the Jacksonburg section, and also frequently occurs 
elsewhere. The New Jersey specimens differ from the Cincinnatian 
shells, which are referred to this species in being generally longer 
in proportion to the breadth, some of the New Jersey specimens being 
nearly subcircular in outline. When imperfectly preserved, it is 
frequently difficult to separate specimens of this species from Dinorthis 
pectinella, but in complete specimens the pedicle valve of P. plicatella 
is much more convex and the cardinal area slopes backward much 
more than in D. pectinella. The brachial valves of the two species, 
as preserved in the New Jersey rocks, are frequently wholly indis- 
tinguishable. 
PLATYSTROPHIA BIFORATA (Schl.). 
Plate IX., Figs. 25--28. 
1895. Platystrophia biforata W. & S., Pal. Minn., pt. I., p. 455, pl. 
Bo igs odes 4 } 
Deseription.—Shell subequally biconvex, with strong fold and sinus, 
wider than long, the breadth ranging from 10 to 20 mm. Hinge-line 
equal to or a little shorter than the greatest width of the shell. 
Cardinal angles rectangular or a little obtuse. Lateral margins 
straight or slightly convex at first, then curving gently into the 
regularly-rounded front. Surface of each valve marked by from 
thirteen to eighteen strong, angular plications, of which three or 
four occupy the fold and sinus. Pedicle valve most prominent on 
the umbo, more or less flattened towards the cardinal extremities ; 
beak incurved and pointed; cardinal area well defined; sinus reach- 
ing nearly to the point of the beak, where it is occupied by a single 
plication, sharply defined by an angular plication on each side, deep 
and rounded anteriorly, where one or two additional plications are 
added on either side of the initial central one. Brachial valve with 
its greatest elevation in the centre, more or less flattened toward the 
, cardinal angles; cardinal area nearly equal to that of the pedicle 
valve; the fold prominent and sharply defined. 
