BRACHIOPODA. 
295 
Fig. 213. Dielasma bovidens, Morton. 
An enlargement of the umbonal portion of 
the brachial valve; showing the slightly 
thickened processes on eitlier side of the 
beak. (c.) 
encroaching upon the umbo and often becoming very oblique to the longitudinal 
axis; with the increase of this obliquity the deltidial plates are thickened in 
their inner surface, which thus becomes more or less protruded. The inverted 
sheath or collar within the foramen is highly developed and clearly shown on 
internal casts. On the interior the dental plates are conspicuous, as in Cryp- 
TONELLA, but they stand vertically upon the bottom of the valve, not showing 
the convergence and often actual union occurring in that genus. 
In the brachial valve the dental sockets are quite deep and narrow, the soc¬ 
ket-walls rising abruptly, though not attaining the height of the dental plates 
of the opposite valve. They are distinctly separated from the crural plates or 
margins of the hinge-plate, and converge toward 
the apex where they merge into a slightly ele¬ 
vated cardinal process; the latter usually appear¬ 
ing as a crescentic submarginal wall, though when 
best preserved is seen to be composed of two 
lateral, somewhat rounded lobes. The crural 
plates are two divergent vertical lamellae, originating just below the cardinal 
process, and attaining a length equal to the distance between their extremities, 
which is about one-third the width of the valve at that point. Between these 
plates lies the long shallow hinge-plate, which is raised but little above the 
bottom of the valve, and is sometimes 
actually adherent to it. This plate 
attains its greatest width at the ex¬ 
tremities of the vertical crural plates, 
its margins converging thence ante¬ 
riorly, its full length often equaling 
one-third that of the valve. To this 
Fig. 2U. Dielasma elongatum, Schlotheim. 
plate are attached all the muscles of interior of the umbonal region of the two valves; 
showing the highly developed dental plates (d), the elon- 
the brachial valve, the scars of both sate, sessile lUnge-plate with its muscular scars, and the 
^ form and mode of attachment of the brachidium. 
anterior and posterior adductors being (davidson.) 
frequently clearly defined upon its surface. Upon comparison of this structure 
with that of Cryptonella the homologies are at once apparent, but there is a 
