LOWER HELDERBERG ROCKS. 227 



Rliynclionella nncleolata (n.s.). 



Plate XXXI. Fig. 1 & 2. 



Shell varying from spherical to spheroid-pentagonal or subpentagonal. 

 Ventral valve convex or depressed convex, abruptly deflected towards 

 the margins : beak small, depressed, closely incurved over that of the 

 opposite valve, often subangular on its lateral margins. Dorsal valve 

 larger, sometimes very gibbous, often a little depressed towards the 

 beak : beak never prominent. 



Surface marked by fifteen to twenty-three simple rounded plications, 

 about four or five of which are slightly elevated towards the front of 

 the dorsal valve into a mesial prominence, and three to five depressed 

 on the ventral valve, so as to form a more or less distinct sinus, which 

 never extends beyond the middle of the shell. These depressions are 

 prolonged in front into a more distinct linguiform extension fitting 

 into a corresponding sinus in the front of the opposite valve, and 

 sometimes curved inwards beyond the plane of a right angle with the 

 back of the valve. 



This species is perhaps more nearly related to Rhynchonella{Terebratula)wilsoni, 

 than any other species in the rocks of New- York. Indeed the analogy between this 

 one and some of the forms referred to that species is so great, that until the limits 

 of variation to which it is subject are better defined, it is scarcely possible to point 

 out characters by which they can always be distinguished. 



The species under consideration differs from authentic specimens of R. wilsoni, 

 from Dudley, Ejigland, in being uniformly more coarsely plicated, and usually 

 more angular in outline. When compared with specimens of the same species from 

 Bohemia, these differences are not so conspicuous. 



An analogous or representative species, from the same geological position in 

 Tennessee, has finer plications, of which, six, seven or eight are often elevated on 

 the front of the dorsal valve, while there are frequently as many as twelve or 

 fourteen on each side of the mesial sinus. 



This species corresponds almost precisely with the English specimens which come 

 to us labelled " Terebratula loilsoin." 



It is possible that the figures 1 a,b, c of Plate xxxi are distinct from those which 



