SPIRIFER^ OF THE HAMILTON GROUP. 219 



DoKSAL valve moderately convex ; the mesial fold strongly defined, wide 

 below and narrow above, flattened and sometimes depressed upon the 

 summit ; the lateral portions of the shell curving gently to the sides, 

 with the cardinal extremities flattened. 



Surface marked by from sixteen to eighteen comparatively wide flat- 

 tened or low rounded ribs on each side of the fold and sinus ; the 

 exterior ribs fine and obscure. In exfoliated specimens, and sometimes 

 in others, the ribs are subangular. The entire surface ( including the 

 mesial fold and sinus, and particularly the intervals between the pli- 

 cations) marked by fine continuous radiating striae ; while faint con- 

 centric striae are visible on the fold and sinus, as well as obscurely on 

 the costae, and more decidedly upon the cardinal extremities ; but 

 these are subordinate to the radiating striae. 



The casts show the muscular area of the ventral valve to be elongate, 

 and margined above by long dental plates which are comparatively strong 

 as shown in figure 8. In another specimen, figure 9, there has been an 

 elevated median crest in the muscular area. 



The figures 1 and G of Plate 35 illustrate the prevailing character. 



In general aspect this species resembles S. medialis ; but it is more gibbous 

 and proportional I}- less in width than the young of that shell, while the radiating 

 striie arc a distinctive feature. The area also is more arcuate than in any forms of 

 S. media Its. 



It is with hesitation that I propose a new species of SpmiFERA, while I am com- 

 pelled to unite several that have hitherto been regarded as distinct ; but in this 

 one the characters seem to be distinguishing, and irreconcilable with any of the 

 species heretofore described. 



Geological formations and localities. In the shales of the Hamilton group, on 

 the east side of Skaneatcles lake, Owasco lake, Borodino, and in loose masses of 

 the rock near Tully, Apulia and other places in Onondaga county. It is not at 

 present known from more western localities. 



