PLATE 25. 



f 



Fig. 1. 470. 1. Pentamerus oblongus. ( Pag. 79.) 



1 a. Ventral view of a perfect specimen, the form being somewhat depressed, and slightly indicating the tri- 

 lobate character of the base. 



1 h. The dorsal valve of a larger specimen, somewhat trilobate in form. 



1 c. The dorsal valve of a smaller specimen, more distinctly trilobate. 



1 d. The ventral valve less elongated and proportionally wider than the prevailing forms. 



1 e. The dorsal valve of a small specimen, having the trilobate form, with an unusually thick and prominent 

 beak. 



1 /. A ventral valve nearly circular in form, being somewhat wider than long. 



1 g-. A small dorsal valve of nearly circular form, excepting the prominence of the beak. 



\h. K small ventral valve of circular form. 



1 i. A profile view of a small perfect specimen. 



1 fc. The interior of two fragments of dorsal valves, showing that the single plate is in etfect couiposed of 

 double laminas, produced by the folding inwards of the inner lamina of the shell. 



1 I, ra. Dorsal and ventral view of a silicified cast of this species, showing in a very satisfactory manner the 

 internal structure. 



