SPIRIFER^ OF THE UPPER HELDERBERG GROUP. 195 



In some of the larger or older individuals, the plications are low and gently 

 rounded : in other specimens they are more prominent, while from exfoliation they 

 often become angular and more conspicuous ; and the same appears to be true of 

 the dichotomous plications of the mesial fold and sinus. In two or three instances, 

 I have noticed in the ciists a partial bifurcation of one or two of the lateral plica- 

 tions. In some of the casts, or partial casts, the plications on the fold or sinus 

 appear to be partly or entirely obsolete. The interior of the valves is unknown. 



The specimens before me vary from less than three-fourths of an inch in height 

 and Avidth, to an inch high b}' an inch or an inch and a half wide. I have seen a 

 single specimen two inches wide, and another of more than an inch and a half in 

 height and width. The illustrations, from figure 17 to figure 23 on plate 28, are 

 of the prevailing forms observed. Figure 29, plate 27, is of a cast from the Scho- 

 harie grit. 



Geological formations and localities. This shell occurs rarely in the Schoharie 

 grit in the eastern part of the State, and in the Corniferous limestone at Clarence 

 and Williamsville in the western part of the State. I have seen a specimen from 

 Canada West, and it is known to occur at Sandusky and Columbus, Ohio ; while 

 the better specimens have been obtained from the neighborhood of Dayton, Ohio, 

 and were given me by Judge Gkier of that place. 



Spirifera ^regaria. 



PLATE XXVIII. 



Spiri/er grtgaria : Clapp, MS. 



" . " Hall, in Tenth Report on the State Cabinet, p. 127. 1857. 



Spiri/era gregaria : Clapp, Billings in Canadian J^rnal, p. 128. 1857. 



Shell ventricose, subglobose, semioval or subquadrate in outline : hinge- 

 line equalling or less than the width of the shell ; cardinal extremities 

 truncate or rounded. Surface plicated. 



Ventral valve the more gibbous, regularly arcuate from beak to front, 

 the greatest convexity at or a little above the middle, and curving 

 somewhat abruptly to the sides and more gently to the front : beak 

 much elevated, and the apex closely incurved over the fissure ; area 

 high, concave, and extending to the cardinal angles, where it is some- 

 times more than half a line high, often distinctly striated; mesial 

 sinus rounded or subangular, and much produced in front. 



LoRS.\L valve very convex, with a strong mesial fold, either angular or 

 somewhat flattened along the summit, and sometimes marked by an 



