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. 



' Aiubocoelia uiubonata. 



PLATE XLIV. 



Orlhit umbonata : Conead, Jour. Acad. Nat. Sci. PhiladaJphia, Vol. viii, pa. 264, pi. 14, f. 21. 



" " : Conbad; IIall, Tenth Report on the State Cabinet, p. 167. 1857. 



^mbocalU umbonata : ( Conbad sp.) ; Hall, Thirteenth Report on State Cabinet, pp. 71 & 72. 1800. 

 Orlhit nucleut : Hall, Geo). Report Fourth District, pp. 180 & 181. 

 Ambocalia nucleut : Hall, Thirteenth Report on the State Cabinet, p. 71. 



Shell plano-convex, soniewhat 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 j 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 



