MERISTELLA OF THE UPPER HELDERBERG GROUP. 
311 
In a specimen from the Upper Helderberg limestones of the Falls of the Ohio 
(fig. 32 of pi. l ), the fold is sharp and clearly defined, slightly oblique and 
limited to the dorsal valve, being intermediate in character to the former two ; 
while the specimen is more gibbous than usual. 
I have heretofore proposed (loc. cit.) to designate these varieties of Meristella 
unisulcata, thus recognized, as var. uniplicatd for the specimens from the western 
limestones, and as var. biplicata for the Hamilton group form. 
Geological for nation and localities. This species is not uncommon in the Upper 
Helderberg limestone, in Albany and Schoharie counties, and generally through¬ 
out the extent of that formation within the State ; though nowhere abundant. It 
occurs in the same limestone at the Falls of the Ohio, and in Canada West. In 
the Hamilton group, it has been collected on the shore of Canandaigua lake. Casts 
of the same species have also been observed in the Oriskany sandstone. 
Note. I would, in this connection, call the attention of the student to the illus¬ 
trations of the internal spires of Athyris, Meristella and Meristesta. The 
modification of the parts are confined to the connecting loop of the crura : and 
neither in these, nor in any of the genera of the Spiriferid/B, is there any 
important variation in the form of the spires proper, while there are various 
modifications of the loop ; and I am disposed to believe that others will yet be 
found, perhaps even in the Genus Spirifera itself. These modifications of the 
form and direction of the parts become exceedingly interesting when compared 
with the various forms of the loop shown in the several genera of the Family 
Terebratulid/E, the most of which began their existence upon the waning of the 
spire-bearing forms, or acquired their fullest development in later geological 
epochs where the Spiriferiile are comparatively rare or entirely unknown. This 
modification of S-ura and loop in the later genera is developed more extremely 
in the absence of spires, which acquired their greatest development in medial 
geological times, or about the Devonian or Carboniferous periods, almost at the 
same time that the terebratuloid genera began their existence. 
