SPIRIFER^E OF THE UPPER HELDERBERG GROUP. 
£S3 
Spirifera divaricata. 
PLATE XXXII. 
Spirifer divaricatus : Hall, Tenth Report on the State Cabinet, p. 130. 1857. 
“ “ Description of New Species Pal. Fossils, p. 93. 1857. 
Spirifer venustus : 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, 
