BRAcHIOPODA. 795 
Eumetria, Hall. 1863. 
(Plate 37, figs. 1-12.) 
Shells elongate-terebratuliform; outline ovate. Valves sub- 
equally convex. Hinge-line short ; cardinal area of the pedicle- 
valve somewhat elevated, primarily composed of symmetrical 
deltidial plates. In the adult condition the line of symphysis 
between these plates is lost, or represented by a faint line, giv- 
ing the deltarium the appearance of a single vertical, or slightly 
incurved plate, sharply defined on its lateral margins. The 
foramen is apical, its lower side only encroaching on the del- 
tarium. The cardinal extremities are slightly alate, a feature 
more noticeable on the brachial valve and which gives this valve 
a somewhat pectenoid appearance. On the interior of the pedicle- 
valve the teeth are of moderate size, but otherwise the shell is 
nearly devoid of markings of any kind. There is no apical for- 
aminal tube as in Rerzra, no dental lamellz or muscular ridges, 
and only in extremely rare instances is there any trace of the 
muscular impressions. 
In the brachial valve the structure of the hinge-plate is very 
complicated. It may be described as composed of two parts, a 
posterior and an anterior. The posterior portion is rather 
broadly crescentic in form, having the curvature of the umbonal 
margin of the valve; its lateral extensions form the socket-walls, 
which are moderately broad, deep and well defined. On the cen- 
tral portion of this part of the plate rests a second crescent, hav- 
ing its horns, which make nearly a semi-circle, directed backward 
and into the umbonal cavity of the opposite valve. At this 
point the curvature of the plate is such that the base of the cres- 
cent lies upon the inner surface of the deltarium of the pedicle- 
valve, its horns crossing the deltarium, extending for fully half 
the length of the umbonal cavity, and being elevated, at their tips, 
above the inner surface of the shell. The posterior part of the 
hinge-plate is connected with the anterior part only by a nar- 
row, thickened median band, which is constricted transversely 
at the point of union, the transverse groove being crossed only 
by a very fine axial ridge. The anterior portion of this plate 
consists of a long, narrow, triangular, concave or spoon-shaped 
central process, the edges of which are sharply elevated, and 
the extremities of these marginal ridges are produced into two 
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