SPlRIFERlDiE. 325 



cated by a small round perforation separated from the hinge-line 

 by 3 deltidium. A deep triangular pit or foramen beneath the 

 beak, which is filled b}^ the closely incurved beak of the dorsal 

 valve. False areas sometimes defined. 7 sp. Upper Silurian — 

 Middle Devonian; United States. B. hirsiUa, Hall (cxxxviii, 

 24-27). 



RHYNCHOSPiRA, Hall, 1859. {Etym. — p^yxo^, 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 sj-mmetrically rounded, and with less distinct 

 mesial sinuosities ; and in these characters the}^ 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 or striated. 7 sp. Silurian — Devonian; United States, 

 Russia. R. formosa^ Hall. 



AcAMBONA, White, J 862. 



/Si/?2.— Eumetria, Hall, 1864. 



Distr. — 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 in having a pointed A-^entral 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 A'entral beak and true area. 



Dayia, Davidson, 1881. 



Didr. — D. navicida^ Sowb. (cxxxviii, 28, 29). Silurian; 

 England. 



Oval, broadest posteriorly ; ventral valve A^ery convex, keeled 

 along the middle, beak closely incurved, dorsal valve slightly 

 convex posteriorly, anterior half of shell concave, surface 

 smooth ; in the interior of the dorsal valve a slightly raised 

 ridge extends from under the hinge-plate to about half the 

 length of the valve, and on either side are the two adductor 

 scars ; 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. 



