LOWER HELDERBERG ROCKS. 
Rhynchonella 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 
i 
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 (Terebratiila) 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, England, 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 u Terebratula wilsoni.’’'’ 
It is possible that the figures 1 a, b, c of Plate xxxi are distinct from those which 
