SILURIAN FAUNAS. 231 
slight anterior curvature as they approach the margin. As the 
shells increase in size this peculiar curvature of the ribs becomes 
more and more conspicuous, and can always be detected to a greater 
or less degree in the adult individuals, and it is always more con- 
spicuous in the brachial than in the pedicle valve. Just anterior 
to the beak in both valves there is a small area where the radiating 
ribs are obsolete. In addition to the radiating ribs, both valves are 
marked by exceedingly fine, concentric lines. 
The dimensions of a large individual are: length, 14 mm., and 
width, 22 mm. 
Remarks.—This species is exceedingly abundant in the lower beds 
of the Decker Ferry formation. It may be always recognized, in the 
larger specimens at least, by the peculiar curvature of the radiating 
ribs. Among the smaller, and therefore younger, specimens, however, 
this character is a variable one, the curvature of the ribs seeming to 
start earlier in the life history of some individuals than in others. 
On comparing young individuals of approximately the same size one 
will frequently be seen to possess perfectly-straight ribs, while in the 
other the ribs will be slightly curved. In the large specimens, how- 
ever, the curvature may always be detected. On comparing smaller 
specimens with straight ribs and larger ones with curved ribs the dif- 
ferences seem to be sufficient to indicate distinct species, but as all 
intermediate characters seem to exist, they are all placed together in 
the same species. 
ORTHIS FLABELLITES Foerste. 
Plate XX., Fig. 20. 
1897. Orthis flabellites Schuchert, Bull. U. 8. Geol. Surv., No. 87, 
p. 286. (A full bibliography of this species will be found 
here. ) 
Description. Brachial valve subelliptical in outline, depressed con- 
vex, hinge-line equal to or a little shorter than the greatest breadth 
of the shell; median portion flattened or slightly concave, forming 
a broad, shallow, median sinus, whose centre is scarcely depressed 
below its sides. Surface marked by from thirty to thirty-six strong, 
radiating ribs, between which, near the margin in internal casts, there 
are intercalated some finer ribs. Pedicle valve not recognized in the 
New Jersey collections. : 
