SPIRIFERID A. 325 
cated by a small round perforation separated from the hinge-line 
by a deltidium. <A deep triangular pit or foramen beneath the 
beak, which is filled by the closely incurved beak of the dorsal 
valve. False areas sometimes defined. Tsp. Upper Silurian— 
Middle Devonian; United States. &. hirsuta, Hall (cxxxviii, 
24-27). 
RHYNCHOSPIRA, Hall, 1859. (Htym.—pvyxos, a beak, and spira ; in 
allusion to its similarity in form to Rhynchonella, and having 
internal spires.) Shell somewhat similar to Rhynchonella, but 
usually more symmetrically rounded, and with less distinct 
mesial sinuosities ; and in these characters they resemble Wald- 
heimia. Valves articulated by teeth and sockets, similar to 
those of Nucleospira; the crura supporting two conical spires. 
The cardinal process of the dorsal valve is a broad emarginate 
plate; beak of the ventral valve largely perforated. Surface 
plicated orstriated. Tsp. Silurian—Devonian; United States, 
Russia. £&. formosa, Hall. 
ACAMBONA, White, 1862. 
Syn.—Eumetria, Hall, 1864. 
Disir.—A. prima, White (cxxxviii, 30). Carb.; U.S. 
Shell resembling Retzia externally, 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 extending 
forward of the beak of the dorsal valve on each side of it; beak 
of 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. 
Differs from Retzia i in having a pointed ventral beak, curved 
hinge-line, and no angular cardinal wings on the dorsal valve ; 
from Uncites in having an area and punctate structure ; from 
Trematospira in its pointed ventral beak and true area. 
Daya, Davidson, 1881. 
Distr.—D. navicula, Sowb. (exxxviili, 28, 29). Silurian; 
England. 
Oval, broadest posteriorly ; ventral valve very convex, keeled 
along the middle, beak closely incurved, dorsal valve slightly 
convex posteriorly, anterior half of shell coneave, surface 
smooth; in the interior of the dorsal valve a slightly raised 
ridge extends from under the hinge-plate to about half the 
leneth of the valve, and on either side are the two adducto 
sears; the sockets are widely separate; primary stems of the 
spirals extend parallel to each other for a short distance, bend 
at right-angles abruptly towards the lateral portions of the beak, 
