BRraouiopoDA. 761 
Amboceelia, Hall. 1860. 
(Plate 31, figs. 8-17) 
Shells small, concavo-, or plano-convex. Marginal outline 
nearly semi-circular. Hinge-line long and straight, its length 
nearly or quite equaling the greatest transverse diameter of the 
shell. 
Pedicle-valve greatly elevated; umbo arched and incurved ; 
with a narrow median groove which becomes fainter or disappears 
toward the anterior margin. Cardinal area well defined and 
arched ; divided medially by an open delthyrium whose lateral 
margins bear incomplete deltidial plates. Teeth prominent, erect, 
strongly recurved at the tips; not supported by dental plates. 
Muscular area quite restricted, consisting of narrow, elongate 
diductors, inclosing an almost linear adductor. The entire area 
is sometimes divided by a faint median ridge. The interior sur- 
face about the muscular area is strongly pitted. 
Brachial valve convex at the beak, becoming depressed over 
the pallial region and reflexed near the margin. Cardinal area 
comparatively broad and standing at nearly right angles to the 
area of the opposite valve. Delthyrium open, the deltidial coy- 
ering attaining the same degree of development as in the pedicle- 
valve. Cardinal process narrow and much elongated, resting on 
the bottom of the valve except at its posterior extremity, which is 
simply bifurcated. Crural plates erect, parallel; taking their origin 
in the deltidial plates and extending about one-fourth the distance 
across the valve. The spirals are attached by long crura, the 
ribbon making afew volutions only, thus forming loose coils, 
directed laterally. The jugum has apparently the same incipient 
condition of development as in Sririrer. According to Giuierr, 
the spiral ribbon bears spinules on its outer margins. Muscular 
impressions anterior and composed of four well-defined adductor 
scars. 
Surtace smooth or with fine concentric striz crossed by 
indistinct radiating lines; rarely spinous. Shell substance 
fibrous, impunctate. 
Type, Ambocelia uwmbonata, Conrad (sp.), Hamilton group. 
Distribution. Devonian— Carboniferous. 
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