• PLATE V. 



Fig. 



1. Section of the shell of Terehratula hullata (Oolitic)j near the external surface, and 



parallel to it, showing the remarkably large size and close proximity of the 

 perforations. — Magnified 100 diameters. 



2. Section of the shell of Megerlia lima (Cretaceous), taken in the like direction, 



showing -the very small size and wide separation of the perforations. — Magnified 

 100 diameters. 



3. Section of the shell of Spiriferina rostrata (Triassic), taken in the like direction, 



showing the extremely large size and close proximity of the perforations, and 

 the appearance presented by the oblique division of the flattened prisms of 

 which the shell is composed. — Magnified 100 diameters. 



4. Portion of the shell of Bhynconella psittacea (Recent), showing at a a the internal 



surface, with the imbricated arrangement of the extremities of its component 

 prisms, and oi b b the aspect of the prismatic substance as displayed by a 

 fracture nearly in the direction of the length of the prisms, showing also the 

 entire absence of the least trace of perforations. — Magnified 40 diameters. 



5. Portion of the internal surface of the shell of Bhynconella psittacea, more highly 



magnified, showing the imbricated arrangement, and the entire absence of 

 perforations. — Magnified 100 diameters. 



6. Portion of shell of Bhpiconella octoplicata (Cretaceous), showing the peculiar oblique 



striation of its component prisms, as displayed by a fracture the plane of which 

 passes in the direction of their length. — Magnified 100 diameters, 



7. Internal surface of the marginal portion of the shell of Crania antiqua (Cretaceous), 



as seen under a low power, showing the large orifices of the perforations. — 

 Magnified 17 diameters. 



•'o' 



8. Vertical section of the unattached valve of Crania Norvegica (recent), showing its 

 successive layers, traversed by vertical channels, which present an arborescent 

 division near the external surface, and a marked contraction near the interior ; 



