PLATE 47. 



Page 



Fig. la- m. Pentamerus galeatus. 257 



1 a. A sijecimen having the beak of the dorsal valve broken off, showing the triangular cavity 

 beneath the beak of the opposite valve. 



1 6, c. Ventral valves of this fossil : the one filled vi'ith stone, showing only the triangular 

 cavity a; and the other having the stone removed by weathering, showing the cavity a 

 and the septum n, presenting the common character of this fossil as seen upon the wea- 

 thered surface of the Lower Pentamerus limestone of the Lower Helderberg group. 



1 d. Dorsal valve, showing the entire extent of the cavity formed by the dental lamellae, and 

 the central septum below. 



1 e. Lateral view where portions of both valves are removed, showing the extent of the dental 

 lamellae or spoonshaped cavity a as shown in flg. 1 d, the central septum of the ventral 

 valve B, and one of the septa d of the dorsal valve, which becomes free at its extremity. 



1/. Interior of the hinge : e, e, the line of junction of the two valves. The septa of the 

 upper or dorsal valve correspond to the extension of the dental lamella; or sides of the 

 spoonshaped cavity in the lower or ventral valve. 



1 g. Dorsal view of a cast, showing the septa d, o; the cast of the triangular cavity beneath 

 the beak of the ventral valve a. 



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 longitudinal septa of the dorsal 

 valve are marked D, d, as in the preceding figure. 



1 i. Cast of the ventral valve, showing the longitudinal septum b. 



1 fc, I, m. Profile, ventral and front views of an extravagant specimen which shows bifurcating 

 plications upon the mesial sinus and fold. 



