BRACHIOPODA. 97 
The external surface is covered with numerous fine radiating 
strie, which are rarely crossed by concentric lines. Shell- 
substance richly punctate. 
Type, Petzia vera, Hall. Kaskaskia limestone. 
Distribution. Lower — upper (?) Carboniferous. 
(2?) Acambona, White. 1862. 
(Plate 37, figs. 21, 22.) 
Shells similar to Eumerrta. Beak of pedicle-valve prominent, 
incurved, pointed (?). Deltarium triangular or flat. Brachial 
valve without the alate cardinal extensions of Eumerria. Internal 
pedicle-tube slightly developed. Hinge-plate with two short 
processes on its posterior edge which extend for only a short 
distance into the umbonal cavity. 
Type, Acambona prima, White. Burlington limestone. 
Distribution. Lower Carboniferous. 
Hustedia, Hall. 1893. 
(Plate 37, figs. 13-20.) 
Shells distinguished externally from Eumerria by their much 
coarser plications. The essential difference from that genus lies 
in the structure of the hinge-plate and the umbonal cavity of the 
pedicle-valve. The latter contains an internal tube attached by 
one side to the deltarium and split along the opposite side, a 
precisely similar structure to that observed in Rerzia and 
Acampona, though not so highly developed as in the first of 
these. The hinge-plate is constituted as follows: It is erect and 
recurved into the umbonal cavity of the pedicle-valve, projecting 
considerably beyond the hinge-line; the upper 
face is convex and elevated medially, the 
posterior margin sinuate and crescentic, 
though the horns of the crescent are very © Oy 
short ; two deep converging grooves pass over 
the upper face, and outside of these on the aie Ree ae 
lateral margin of the plate are strong lobes moni, Marcou (ep ),withen- 
which bear the erect, slightly recurved (2700 trausverse sepions 
crura; from the crural bases the lateral foramen; showing the in- 
margins curve downward to the bottom of ore eee tence nt 
the valve and form the socket walls. At the plates. 
base of the cardinal process and in the median line arises a 
free, slender, ligulate process which curves upward and back- 
49 
