AMBOCCELIA OF THE HAMILTON GROUP. 
259 
bounding the teeth-sockets. The cardinal process is elongate, lying be¬ 
tween the crura, and is distinctly bifurcate or bilobed at the outer ex¬ 
tremity as in Cyrtina. The muscular impressions are below the middle 
of the valve, often near the front, and are usually distinctly quadruple. 
The dorsal valve being depressed-convex, flat or concave, the spires lie 
in the cavity of the ventral valve : no peculiarity has been observed in 
those appendages. 
With these remarks, I shall for the present leave the following species 
under the designation of Amboccelia. 
Amboccelia umbonata. 
PLATE XLIV. 
OrtJiis umbonata : Conrad, Jour. Acad. Nat. Sci. Philadelphia, Vol. viii, pa. 264, pi. 14, f. 21. 
“ “ : Conrad; Hall, Tenth Report on the State Cabinet, p. 167. 1857. 
Amboccelia umbonata : (Conrad sp.) ; Hall, Thirteenth Report on State Cabinet, pp. 71 & 72. 1860. 
Orthis nucleus : Hall, Geol. Report Fourth District, pp. 180 8c 181. 
Amboccelia nucleus : Hall, Thirteenth Report on the State Cabinet, p. 71. 
Shell plano-convex, somewhat semiorbicular or obliquely subconical, 
rather wider than long; hinge-line equal to the width of ' the shell or 
rarely a little less, the cardinal extremities rounded. 
Ventral valve gibbous, with the umbo extremely elevated and-a com¬ 
paratively large incurved beak : mesial sinus distinct, extending from 
the beak to the front of the shell ; the area rather large, arched and 
of considerable height, extending conspicuously to the cardinal ex¬ 
tremities. The fissure is partially arched over by a pseudo-deltidium. 
Dorsal valve semielliptical, depressed-convex in the upper central por¬ 
tions, concave below the middle and at the sides : apex scarcely ele¬ 
vated above the hinge-line ; area equalling the thickness of the shell. 
There is no perceptible mesial fold, furrow or impressed line in this 
valve. 
Surface marked by very fine radiating and concentric striae; the latter 
sometimes becoming crowded, lamellose and imbricating towards the 
front of the valves. 
The interior of the ventral valve shows a comparatively strong tooth 
on each side at the base of the fissure, which is continued in a callosity 
