LOWER HELDERBERG ROCKS. , 239 



Fig. 1 A. 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 t. Cast of the ventral valve, showing the longitudinal septum B*. 



Fig. 1 k, I, 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 Shaly 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. 



Pentamerus pseudogaleatus. 



Plate XLVI, Fia. 2a-/. 



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. 



