BRACHIOPODA. 
105 
Shell elongate-oval, rather broad over the pallial region. Surface covered 
with rather coarse, angular, usually simple plications. There is a trace of an 
indistinct median sinus on the pedicle-valve in which the plications are slightly 
smaller than those adjoining. The umbo of the pedicle-valve is incurved and 
its apex truncated by a circular foramen. The deltidium is triangular, flat, or 
arched by the incurvature of the beak; the deltidial plates are firmly anchyl- 
osed into a single piece and the original line of symphysis is represented by a 
thickened ridge. The edges of the cardinal area are well defined, but not alate 
on either valve, the beak and area of the brachial valve being entirely concealed 
by incurvature. The cardinal slopes are broad and smooth. On the interior of 
the pedicle-valve the teeth are rather small, and are supported by thin lamellae 
which traverse the umbonal cavity and rest on the bottom of the shell. These 
lamellae are produced forward for a short distance, limiting, posteriorly, the 
muscular area. The apical portion of the umbonal cavity contains a longitud¬ 
inal tube attached by one side to the inner surface of the deltidium. Just 
within the outer opening of the foramen this tube appears to have been closed 
on all sides, but further toward the cardinal margin it becomes split along the 
back or outer surface, diminishing in size downward and disappearing entirely 
before reaching the hinge-line. (In the accompanying figures of transverse 
sections, 80-83 are from a single specimen, 84, 85 from another, and 86-98 
from a third.) In sections made across the vertical foramen it is seen that 
the tube extends within the deltidium and forms a subcircular enfolding of 
testaceous matter from the margins of the foramen. This organ is similar to 
that elsewhere described in the genera Hustedia and Acambona, but it appears 
to be more highly developed and longer in Retzia than in either of these. 
