FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 121 



tions upon the fold or sinus, giving six to eight at the margin of the shell. I 

 have before me a specimen of grieri, in which the first rib on each side of 

 the mesial fold and the second one on each side of the sinus are plainly dicho- 

 tomous. In perfect specimens the surface is covered by fine concentric lamel- 

 lose striae, which are crossed by delicate, radiating striae. This species is 

 distinguished from most of the allied forms by its simple, strong plications 

 on each side of the mesial fold and sinus, while those occupying the latter 

 are smaller and bifurcating. 



Sometimes the middle plication on the mesial fold is simple, in which case the 

 fold is quite angular, while in other instances it bifurcates, leaving a longitu- 

 dinal depressed line on the middle, giving it a more rounded outline. It is 

 only on specimens which have suffered no injury by wearing or exfoliation, 

 that the fimbriate appearance of the concentric markings is visible. In some 

 of the larger and older individuals the plications are low and gently rounded, 

 in others they are more prominent, while from exfoliation they often become 

 angular and more conspicuous, and the same appears to be true of the dicho- 

 tomous plications of the mesial fold and sinus. The interior of the shell is 

 so far unknown. (Hall.) 



The size of the shell differs considerably in the different specimens ; its 

 width is from less than three-fourths of an inch to an inch and a half, and its 

 length measures from less than three-fourths of an inch to an inch. The speci- 

 men figured on plate 9, figures 8 to 12, has a width of one inch and five- 

 eighths, by a length of an inch and a quarter, but specimens of these dimen- 

 sions are of unusually large size, and of rare occurrence. 



Formation and Locality. This species occurs in the Corniferous limestone at and near the Falls of 

 the Ohio, in Indiana and Kentucky; it appears in some strata pretty abundantly, though good and well 

 preserved specimens are somewhat rare. This species was named in honor of Judge Grier, of Dayton, 

 Ohio, who presented Prof. Hall with good specimens from that locality. 



Spirifera hobbsi. N. SP. 



Plate X., figures 21, 22, 26 to 30, and 35 and 40. 

 Spirifera varicosa var., Hall. Pal. N. Y., Vol. 4, p. 206, pi. 31, fig. 23. 



Shell transverse, semi-oval ; width exceeding the length considerably, some- 

 times having its double size ; hinge-line equalling the greatest width of the 

 shell, and terminating in salient, mostly mucronate angles. Surface plicate. 



Ventral valve much elevated, sub-pyramidal ; most prominent at the beak 

 or a little in front of it. The beak is not incurved, or at least only slightly 

 so ; mesial sinus well defined and forming a deep triangular groove, with an 

 acute angle in the bottom, resembling that of Spir. byrnesi, but not so much 

 produced at the front. Cardinal area straight and large, with sharply angular 



OEOL. SUK. 1C 



