LOWER HELDERBERG ROCKS. 
SOS 
Spirifer modestus (n. s.). 
* Plate XXYIII. Fig. 1 a - e. 
Shell small, subglobose. Ventral valve very gibbous near the middle and 
towards the beak, having a shallow undefined sinus extending from 
the beak to the front : beak prominent, acutely pointed, incurved. 
Dorsal valve regularly convex, semicircular or subtriangular : extre¬ 
mities rounded, sometimes an undefined mesial elevation : beak scarcely 
extending above the hinge line, not incurved; hinge line very short, 
rounded at the cardinal extremities. Area triangular, faintly defined, 
about half the width of the shell, arcuate. Foramen of medium size, 
narrow, triangular. Dental lamellae slightly diverging, and extending 
more than half way to the base of the shell. 
Surface marked by faint concentric lines of growth. 
This species resembles somewhat JVucleospira ventricosa of the Lower Helderberg 
shaly limestone, but has a much higher area : it still more resembles S. lineatus of 
the Carboniferous system, but differs in its surface markings and its narrower fo¬ 
ramen. 
Fig. 1 a. Dorsal view of specimen of ordinary size. 
Fig. 1 b, c, d. Ventral, dorsal and profile views of a larger individual. 
Fig. 1 e. The interior of the veptral valve. 
The margins of the area of this and the preceding species are usually prominent or exsert. 
Geological position and locality. Limestones of the Lower Helderberg group : 
Cumberland- (Md.). 1 
Plate XXVIII. Fig. 2 a-/. 
Shell rhomboidal ventricose, a little wider than high : valves nearly 
equally convex. Hinge line about equalling the width of the shell. 
Ventral valve much elevated and extended towards the beak, which 
is slightly curved at the apex : sinus subangular, becoming deep 
towards the front and much produced. Area twice as wide as high, its 
width being about half the width of the shell. Ventral valve gibbous in 
* 
