448 MISSISSIPPIAN BBACHIOPODA 



Each valve is marked by about 22 sharply angular, nearly uniform 

 simple, radiating plications which increase gradually in size in passing 

 from the beak towards the margin. Conspicuous growth lines are obsolete 

 upon the type specimen, but exceedingly minute, concentric striae may be 

 detected upon some portions of the shell. The shell structure is minutely 

 punctate. 



Remarks, This species differs primarily from the other recognized 

 members of the genus in its much coarser and deeper, strongly angular 

 plications. The shell is also proportionally broader than is usually the 

 case with individuals of other species, it being nearly equal in length 

 and breadth. 



Horizon. Chester group. 



Genus AC AMBON A White 



Description. Shells of medium size or larger, subovate in outline, the 

 hinge-line very short, the valves subequally convex, marked by fine, 

 rounded, simple plications, the shell structure minutely and closely 

 punctate. Pedicle valve with a small cardinal area having its lateral 

 margins sharply defined and elevated above the surface of the valve on 

 either side, the delthyrium occupying nearly the entire cardinal area, 

 closed by the pseudodeltidiurn ; the foramen large, subcircular, encroach- 

 ing wholly upon the umbonal region of the valve. Internally, dental 

 lamellaa are apparently absent, no other internal characters recognizable 

 in the genotype except the presence of a spiral brachidium. The brachial 

 valve slightly compressed at the cardinal extremities to form minute 

 auriculations, the internal characters not preserved in the genotype. 



Remarks. The essential generic characters of the holotype of A. prima, 

 the genotype of Acambona, are not preserved, and so far as this specimen 

 is concerned, there is no reason for placing it in any genus other than 

 Eumetria. Hall and Clarke 1 have stated, however, that members of the 

 species A. prima bear "an internal pedicle-tube, as in Retzia and Hustedia, 

 a character absent in Eumetria." Specimens of this species are exceedingly 

 rare and only the holotype has been available to the writer for study, 

 and no such pedicle-tube is visible in that specimen, although it may be 

 present. Until further information concerning the genus can be secured 

 it seems best to allow it to stand with the genotype as the single repre- 

 sentative, although if it is really a good genus, it is possible that some 

 of the species here placed in Eumetria may be eventually transferred to it. 



1 Pal. N. Y., vol. 8, pt. 2, p. 119. (1894.) 



