MERISTELLA OF THE UPPER HELDERBERG GROUP. 299 



At the same time, an examination of the Niagara species referred by me to the 

 Genus Meristella presents a difi'erent condition of the interior, and shows the 

 lamclhe of the spires united by a simple loop only. 



This feature is illustrated in the accompanying diagram, which represents a single turn 

 of the spiral lamellae, fro;n their origin at a, to i where the bands are cut off. The thickened 

 bases of the crura are represented at a; b is the point of recurvation, c the continuance of 

 the lamellae in the dorsal cavity, and d the expansion on the inner side into long processes 

 vhich unite at e, forming the loop.* 



Meristina MAEIA i 



Showing the first volution of the spiral lamellae. 



The same characters have also been observed with equal distinctness in M. nitida of the 

 Niagara group. In neither of these species is there any indication of accessary lamellaj as in 

 Atuyuis, nor evidence of the extension of the loop beyond the point e; and we are there- 

 fore induced to believe that the simple character of the spires in these forms will constitute 

 another distinction, which may conveniently be termed Meristina. 



91ciistclla nasata. 



PLATE XLVIII. 



■Alrijpa nasnta ; Conrad, Ann. Report on the Palaeontology of New-Tork, p. 18. 



Meristella natuta (Conrad sp.) : Hall, Thirteenth Report on the State Cabinet, p. 93. 1860. 



MerUtella elism : Hall, Fourteenth Report on the State Cabinet, p. 100. 1861. Fig. 21 & 22, Fif- 



teeuth Report. 1862. 



MerUtella natuta : Hall, Fifteenth Report on the State Cabinet, p. 100 i figs. 17-19, p. 161. 1862. 



jllhyiis clara : BiLLiNos, Canadian Journal, p. 274. 1800. 



Jthyrit Clara : Geology of Canada, p. 378, flg. 397. 1803. 



Shell suboval, ovate or subrhomboidal, the greatest width near or a little 

 below the middle ; the length equalling or greater than the width. 

 Valves convex, the ventral valve gibbous. A nasute or linguiform 

 extension of the front in old shells. 



• The point of junction in these parts varies in different species ; and even in what appear to be the 

 same species, the points of Jimction arc not id.!utical in all individuals. 



