BRAcHIOPODA. 823 
tinuance, and we must, therefore, be prepared for closer dis- 
criminations in this great group of species than have elsewhere 
been necessary or advisable, and to emphasize such of these 
deviations from this stable line of development, as are justified 
by their persistence and the convenience of classification. 
‘Rhynchonella, sensw stricto. 
(Plate 42, figs. 1-3.) 
Subpyramidal shells having the margins of the valves sinuous 
or angulated. Pedicle-valve with a median sinus beginning in 
front of the convex umbo and, in the type-species, becoming 
broad and deep, producing a prominent linguiform extension at 
the anierior margin. Brachial valve convex in the umbonal 
region and developing anteriorly a prominent median fold. 
Surface of both valves more or less plicated, often accompanied 
(as in the type) by fine concentric lines of ornament. The apex 
of the pedicle-valve is but slightly incurved and exposes a cir- 
cular or elongate-oval foramen enclosed by the deltidial plates 
beneath, and above by the substance of the valves. Here isa 
narrow pseudo-area defined by oblique cardinal ridges diverging 
from the beak. On the interior the teeth are well developed and 
are supported by lamellz which rest on the bottom of the valve 
near the beak, but are free anteriorly. The muscular area con- 
sists of a moderately deep oval scar extending one-third the 
length of the valve, and composed of two large diductors com- 
pletely enclosing small central adductors. The posterior surface 
about the muscular area is pitted with ovarian markings. 
In the brachial valve there is no cardinal process; the crural 
plates are simple, divergent, somewhat expanded on the upper 
surface but not conjoined except where they converge beneath 
the beak and meet the median septum, which extends for about 
one-half the length of the valve. The crura are long and curved 
upward toward the opposite valve. Muscular area elongate sub- 
quadrate, with small posterior and large anterior adductor scars 
Shell-structure fibrous. 
Type, Rhynchonella loxia, Fischer de Waldheim. 
Distribution. Jurassic. Cretaceous. 
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