LOWER I-IELDERBERG ROCKS. 
225 
Surface ornamented by twenty-eight to thirty-two simple rounded pli¬ 
cations, broader than the depressions between. On the dorsal valve the 
central depression is a little deeper than the others, extending quite 
to the beak : fine concentric lines, which arch a little upwards, cross 
the plications. 
This species may be compared with Terebratula haidingeri of Barrande, to some 
varieties of which it bears considerable resemblance : it is, however, generally less 
gibbous, and the beak of the ventral valve is much less prominent. The concentric 
undulations, marking the stages of growth, are likewise more faint than in Bar- 
rande’s species. 
I have some doubts whether this fossil is a true Rhynchonella; but the species is 
extremely rare, and has thus far afforded me no opportunity of studying the interior 
structure. 
Fig. 2 a, b , c. Young individuals of this species. 
Fig. 2 d, e, f & h. Dorsal, ventral, profile, and front views of a larger individual. 
Fig. 2 i. Enlargement of the concentric striae. 
Fig. 3 a, b, c. An individual having a shorter and more rotund form, with a faintly marked 
sinus in front. This is probably a distinct species. 
Geological position and locality. In the lower pentameras limestone of the Lower 
Helderberg group : Helderberg mountains, Albany county. 
Hhynclionella mutabiiis (n. s.). 
Plate XXIX. Fig. 4; and Plate XXX. Fig. 1 & 2. 
Shell varying from ovate to spherical. Ventral valve sometimes depressed, 
generally most convex in the umbonial region : beak small, pointed, 
closely incurved over that of the opposite valve. Dorsal valve gibbous : 
beak incurved beyond the hinge line; cardinal border on each side of 
the beak, concave. 
Surface marked by twenty to twenty-six depressed rounded simple 
plications, of which about six or eight are slightly raised towards the 
front of the dorsal valve into an indistinct mesial elevation; and five 
or six depressed near the front of the ventral valve, and extended 
into a short linguiform prolongation ; concentrically marked by fine 
undulating strise ? 
[ Paleontology III.] 29 
