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 

 unisulcala, thus recognized, as var. uniplicata 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 Oriskauy sandstone. 



Note. I would, in this connection, call the attention of the student to the illus- 

 trations of the internal spires of Athyris, Meristklla and JV^eiustina. 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 Spikiferid^, 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 Spirifeea 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^, 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 Si'irifeiud.e are comparatively rare or entii-ely unknown. This 

 modification of crura 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. 



