484 PALEONTOLOGY OF NEW-YORK. 



GEXIS RllYMClIOSPIRA (Hall). 



[ Gr. pvyx<K, rostrum ; amtpa, spira ; in allusion to its similarity in form to RilYNCHO- 



NELLA, and having internal spires.] 



J^rebratula and RkynchoneUa of "authors. 

 WMhtimia : IIai.i., 1850. 



lyemntospira, Subgenus Jihynchotpira : Hall, 1857. 

 Rhynchospira : Hall, 1858. 



Shell longitudinally ovate or subglobose, more or less gibbous, acute at 

 the apex. Valves subequally convex ; mesial fold not strongly defined, 

 one, two, or more smaller plications usually marking the centre of each 

 valve : beak of the ventral valve perforate, the perforation generally 

 well defined, the lower side formed by a deltidium which separates it 

 from the umbo of the opposite valve. 



SuBFACE radiatingly plicate or striate :■ shell-structure fibrous or fibro- 

 punctate ? 



Valves articulating by teeth and sockets; the crura supporting two 

 conical spires, which occupy the greater part of the cavity of the two 

 valves. The cardinal process of the dorsal valve is a broad subemargi- 

 natc plate, spreading laterally and a little recurved at its basal margins, 

 where it is clasped by the teeth of the opposite valve, and extends 

 beneath the deltidium, lying close against the inner surface of that part 

 of the ventral valve. 



The mode of articulation, as now determined, is very similar to that of Nucleo- 

 spira; but the cardinal process is proportionally shorter and emarginate at the 

 extremity, the perforation of the beak large and distinct, while the form is different 

 and the exterior surface plicate or striate, and not pvmctate as in that genus. 



The form of the species is not unlike Rhynchonella, but usually more symmetri- 

 cally rounded, and with less distinct mesial sinuosities. In these characters they 

 resemble Waldheimia, to which genus I had originally referred them until the 

 discovery of the internal spires. 



Several of these shells bear a close resemblance, both in the general form and in 

 the interior spires, to Retzia; but the dorsal valve never presents the straight ex- 

 tended hinge-line, nor the ventral valve the short area, common to the carboniferous 

 species of that genus. 



