118 PALAEONTOLOGY OP KENTUCKY. 



course, moderately widening, but not, or very little, increasing in depth towards 

 the front ; flat at the bottom, and having only a small basal extension, which 

 is not deflected ; umbo more or less prominent, according to age, and beak 

 strongly arched over the umbo of opposite valve ; hinge -area small and unde- 

 fined in its margins, which are rounded and coalesce with the surface of the 

 valve ; triangular fissure of moderate size, with its base to its height as two to 

 three. 



Dorsal valve of equal convexity with the ventral, both in young and old 

 shells : greatest convexity below the umbo, from where it slopes very abruptly 

 to the cardinal margin, but in a gentle and regular curve to the lateral and 

 basal margins ; a mesial fold of moderate elevation and moderate width, but 

 well defined in its whole course, extends from beak to front, with a flat or 

 broadly rounded summit ; umbo moderate, and beak strongly incurved into 

 the foramen of the opposite valve. Surface ornamented by five to six broadly 

 rounded plications on each side of the mesial fold or sinus, of which only 

 the first one, adjacent to the median elevation or depression, is of moderate 

 size and plainly marked from beak to front, while the others diminish in size 

 and distinctness more and more toward the lateral margins. In well preserved 

 specimens retaining the shell, the surface is covered with fine, thread-like, 

 radiating striae, which increase greatly towards the front by bifurcation ; these 

 striae are crossed by fine imbricating lines of growth, which become somewhat 

 crowded near the front. This species is generally found as exfoliated casts, 

 which do not show these radiating and concentric lines, or at least indicate 

 them only very faintly. 



Professor Hall has repeatedly, but always with the expression of doubt, 

 referred this species to Spirifera niagarensis, but it appears to me that it differs 

 too much from that shell to be associated with it. Spir. niagarensis shows 

 in all of HalFs figures large cardinal dimensions ; it has a hinge- line as large 

 as the width of the shell ; it also has a considerable hinge-area, which is well 

 defined, and its plications are numerous and plainly marked ; all these points 

 are greatly different in our shell ; its hinge-line is very short, not exceeding 

 one-half the width of the shell ; the cardinal area is small, and not defined by 

 sharp, angular, but by broadly rounded margins, and the plications are few, 

 and only faintly marked. These differences are certainly sufficient to separate 

 our shell specifically from Spir. niagarensis. Nearer related is this species to 

 Spirifera radiata, from which it differs only by its small cardinal area, which, 

 however, occurs in some specimens of that species also, but mainly by its 

 plicated surface. It certainly occupies an intermediate position between Spir. 

 niagarensis and Spir. radiata, of which it presents the transition-form in the 

 process of evolution. 



Formation and Locality Tt occurs in the Niagara group so prominently exposed in the quarries 



