LOWER HELDERBERG ROCKS. 
2*59 
Fig. 1 h. Cardinal view of the same, showing the form of the cast filling the triangular 
cavity beneath the beak A, and its extension below into the central septum, 
which in the cast divides the valve into two parts. The lines of the two longitu¬ 
dinal septa of the dorsal valve are marked d, d, as in the preceding figure. 
Fig. 1 i. Cast of the ventral valve, showing the longitudinal septum B*. 
Fig. 1 k, l, m. Profile, ventral and front views of an extravagant specimen, which shows 
bifurcating plications upon the mesial sinus and fold. 
Geological position and locality. This species is the characteristic one of the Lower 
Pentamerus limestone of the Lower Helderberg group, and large masses of the 
rock are often made up of the broken and separated valves of this fossil. The more 
perfect specimens are obtained from the Slialy limestone above the Pentamerus 
limestone : Helderberg mountains ; Schoharie, Carlisle, Cherryvalley ; Herkimer 
county ; Catskill, Hudson, and numerous other places in New-York ; Cumberland, 
in Maryland ; Decatur county, in Tennessee, and at numerous intermediate points. 
Fentamems psendogaleafus. 
Plate XLYI. Fig. 2 a - l. 
Shell longitudinally ovate. Ventral valve extremely convex or ventricose, 
especially in the umbonial region : beak very prominent and strongly 
gibbous, incurved, and projecting far beyond that of the other valve; 
front margin, in adult shells, sometimes having a faint mesial promi¬ 
nence. Dorsal valve oval or subcircular, much shorter and more com¬ 
pressed than the opposite : beak incurved; front obliquely flattened, 
or very slightly depressed so as to form sometimes an indistinct sinus, 
often a little produced into a short truncate or rounded extension. 
Surface smooth, or marked by faint concentric lines, and towards the 
margin by stronger wrinkles of growth. 
* These letters do not correspond with those used by Davidson in his Introduction to the Classification of 
the Brachiopoda to indicate similar parts of the shell, simply because this plate, and most of the others of 
the same family, were engraved previous to the reception of Mr. Davidson’s work, and the letters were 
used merely for convenience of reference. 
