SPIRIFER^ OF THE UPPER HELDERBERG GROUP. 313 



Spirifcra divaricata. 



PLATE XXXIT. 



Sfiriftr divaricalut : Hall, Tenth Report on the State Cabinet, p. 130. 1857. 



" " Description of New Species Pal. Fossils, p. 93. 1857. 



Spiri/er tenuttut : Hall, Thirteenth Report on State Cabinet, p, 82. 1860. 



Shell ventricose, somewhat rhomboidal or quadrilateral (looking upon 

 the ventral valve). Dorsal valve semielliptical : hinge-line less than 

 the width of the shell ; cardinal extremities obtuse or rounded ; area 

 large. 

 Ventral valve most convex above the middle, extremely arcuate from 

 the umbo to the base, abruptly curving to the sides : beak abruptly 

 arching over to the area ; sinus plicated, shallow above and becoming 

 rapidly expanded below, with the margins undefined and terminating 

 in a broad triangular extension in front. Area high, flat below, abruptly 

 arcuate above, and reaching to the cardinal extremities : foramen 

 large. 

 Dorsal valve regularly and strongly convex, with an angular mesial 

 fold, which is narrow above and expands towards the front, with bifur- 

 cating plications ; sides regularly curving, and sometimes a little flat- 

 tened towards the cardinal extremities. Area rather wide, with the 

 beak and central portions of the valve arching over it. 

 The surface is marked by numerous fine bifurcating rounded or sub- 

 angular plications ; the mesial sinus having on either side a stronger 

 plication, which bifurcates on one or on both sides. At the beak there is 

 a single plication in the bottom of the sinus, which sometimes continues 

 simple nearly or quite to the base ; while the accessions take place 

 mainly from those on the sides of the depression, till they reach the 

 number of ten, eleven or twelve within the limits of the sinus near the 

 base. In a specimen of ordinary size, where the surface is well preserved, 

 there can be seen sixty or more plications with their divisions at the 

 margin of the shell. In some specimens from the Corniferous limestone, 



