AC AMBON A 449 



ACAMBONA PRIMA White 



Plate LXXVI, Figs. 1-3 



1862. Acambona prima White, Proc, 'Boston Soc. Nat. Hist., vol. 9. p. 27, 



figs. 1-2. 

 ( ?) 1894. Acambona, prima Hall and Clarke, Int. to Study of Brach., pt. 



2, pi. 37, figs. 21-22. 



( ?) 1895. Acambona prima Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 

 51, figs. 40-41. 



Description. Shell above medium size, subovate in outline, longer than 

 wide, the greatest width at about the mid-length, the hinge-line very 

 short, the anterior and antero-lateral margins regularly rounded, the 

 postero-lateral margins nearly straight or slightly convex, meeting, when 

 projected beyond the beak, in an angle of about 67 degrees. The dimen- 

 sions of the holotype, a somewhat imperfect and distorted specimen, are : 

 length of pedicle valve 40 mm., length of brachial valve 35 mm., greatest 

 width 27.8 mm., thickness 21 mm. 



Pedicle valve with its greatest convexity posterior to the middle, ap- 

 parently about half way between the middle and the beak, the median 

 portion of the valve gently convex transversely, the surface becoming 

 more abruptly convex as it approaches the lateral margins, and becoming 

 incurved to the cardinal extremities and to the postero-lateral margins; 

 mesial sinus wanting ; cardinal area very small, scarcely reaching beyond 

 the limits of the delthyrium, sharply defined laterally, its lateral margins 

 sharply elevated from the postero-lateral slopes of the valve, concave 

 with the curvature increasing towards the beak, the delthyrium closed 

 by a pseudodeltidium in its upper part, the lower portion occupied by the 

 beak of the opposite valve ; the beak sharply pointed and strongly in- 

 curved, the foramen apparently large and encroaching entirely upon the 

 umbonal portion of the valve, not in contact with the delthyrium even at 

 its apex. 



Brachial valve apparently about equally convex with the pedicle, with 

 the greatest convexity near or posterior to the middle, the surface curving 

 rather abruptly to the postero-lateral margins, less abruptly to the lateral 

 and more gently to the anterior margin, compressed at the cardinal ex- 

 tremities to form minute alations; mesial fold or sinus wanting; beak 

 pointed and incurved beneath the margin of the pseudodeltidium of the 

 opposite valve and filling the lower portion of the delthyrium of that 

 valve. 



Surface of each valve marked by about 90 simple, depressed, rounded, 

 radiating plications which become obsolete on the umbonal portion of the 

 valves, being clearly defined, indeed, only in the anterior half of each 

 valve, not over fifty of the plications are conspicuous, about 20 on each 



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