FOSSILS OF THE SILUEIAN AND DEVONIAN BOOKS. 113 



Spirifera divaricata. HALL. 



Plate XI., figures 6 to 11, and Plate XII., figures 5 to 11.' 

 Spirifer divaricatus, Hall. Tenth Rep. on the State Cab., p. 130 1857. 

 Spirifer venstus, Hall. Thirteenth Rep. on State Cabinet, p. 82 1860. 

 Spirifera divaricata, Hall. Pal. N. Y., Vol. IV., p. 213, plate 32. 



Shell ventricose, somewhat rhomboidal or quadrilateral (looking upon the 

 ventral valve). Dorsal valve semi-elliptical ; hinge-line less than width of the 

 shell ; cardinal extremities obtuse or rounded ; area large. 



Ventral valve most convex above the middle, extremely arcuate from umbo 

 to base ; abruptly curving to the sides ; beak abruptly arching over the area ; 

 sinus plicated, shallow above and becoming rapidly expanded below, with the 

 margin undefined and terminating in a broad, triangular extension in front. 

 Area high, flat below, but 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 bifurcating plica- 

 tions ; sides regularly curving and sometimes a little flattened towards the 

 cardinal extremities. Area rather wide, with the beak and central portion of 

 the valve arching over it. The surface is marked by numerous fine bifurcating 

 rounded or sub-angular plications ; mesial sinus having on each side a stronger 

 plication, which bifurcates on one or 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 are, sometimes, sixty and more plica- 

 tions with their divisions at the margin of the shell. In some specimens from 

 the Corniferous limestone, where the surface is partially or entirely exfoliated, 

 the bifurcating character of the striae is not observed, and in some specimens 

 the plications appear to have been simple throughout. The plications are 

 crossed by fine imbricating lamellose striae, which are abruptly arched back- 

 wards. (Plate 12, figure 11.) A cast of the ventral valve shows a large oval 

 muscular area, which is deeply divided by a rounded median crest, and 

 strongly striated on the lateral portion. (Hall.) 



The shells represented on plate 12, figures 5 to 11, I was, at first, inclined to 

 consider as specifically different from Sp. divaricata. They appeared more 

 robust, and their plications were fewer and stronger. Instead of a simple 

 bifurcation, as in divaricata, these specimens show some instances where a 

 single plication divided towards the base into three and even four branches, 

 forming a kind of fascicle, which are so characteristic in Spirifera camerata of 



GKOL. SUR. 15 



