LOWER HELDERBERG ROCKS. 233 



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 strias. 



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 pentamerus limestone of the Lower 

 Helderberg group : Helderberg mountains, Albany county. 



Rhynchonella mntabilis ( n. s.). 



Plate XXIX. Fio. 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 striae ? 



{ PAL.fiONTOLOaY III.] 29 



