27 



All these are from the rocks at Burlington, Iowa. 



Fig. 1. Fig. 2. 



ACAMBONA PEIMA, ACAMBONA PRIMA, 



{side view. J (dorsal view. J 



Genus Acambona (n. g.) 

 '^X7], a point ; Afi^av, umbo. 

 Generic description. 



Shell of the general appearance and surface characters of Retzia; 

 furnished with internal spires, pointing outward and downward (?). 

 Beak of ventral valve prominent, incurved, pointed ; area emarginate 

 in front, or V-shaped, reaching to the point of the beak, and extend- 

 ing forward of the beak of the dorsal valve on each side of it. Beak 

 of the dorsal valve closely incurved, filling, or nearly filling, the 

 forked space or emargination in the front part of the area, being 

 itself without angular, winged extensions, or area, to meet that of 

 the opposite valve. 



Shell structure punctate. 



The punctate structure and internal spires fully separate this 

 genus from all the Rhynclionellidce ; its punctate structure, curved 

 hinge line, and general form, separate it from the usual forms of the 

 Spirifidce ; from Retzia, which it most resembles, it differs in having a 

 pointed ventral beak, curved hinge line, and no angular cardinal 

 wings on the dorsal valve ; from Uncites it differs in having an area 

 and punctate structure ; from Trematospira it differs in its pointed 

 ventral beak and true area ; and from Stringocephalus, in its internal 

 spires, gibbous dorsal valve, proportionally small area, and want of 

 ventral septum. 



Acambona prima (n. s.) Shell ovate in outline, regularly rounded 

 in front ; dorsal valve most convex ; beak somewhat narrow, closely 

 incurved ; beak of ventral valve prominent, strongly incurved and 

 sharply pointed ; area small, distance across the forward points about 



