268 



HISTORICAL PALAEONTOLOGY. 



The Lamp-shells (Brachiopods) have now reached a further 

 stage of the progressive decline, which they have been under- 



going ever since the close of 

 the Palaeozoic period. Though 

 individually not rare, especially 

 in certain minor subdivisions 

 of the series, the ' number of 

 generic types has now be- 

 come distinctly diminished, the 

 principal forms belonging to 

 the genera Tcrcbratuia, Tere- 

 bratdla (fig. 194), Terebratidina^ 

 Rhynchonella, and Crania (fig. 



193. A small fragment of Escharhui lOS). Ill tllC last mentioned 

 Oceani, of the natural size ; and a portion r i ^i i 11 i j 



of the same enlarged. Upper Greensand. OI tllCSC, the shell IS attached 



to foreign bodies by the sub- 



stance of one of the valves (the ventral), whilst the other or 

 free valve is more or less limpet shaped. All the above-men- 



Fig. 194. Terebra.tclla Astier'ana. Cault. 



tioned genera are in existence at the present day; and one 

 species namely, Terebratidina striata appears to be undis- 

 tinguishable from one now living the Terebratidina caput- 

 serpentis. 



Whilst the Lamp-shells are slowly declining, the Bivalves 

 (Lamellibranchs} are greatly developed, and are amongst the 

 most abundant and characteristic fossils of the Cretaceous 

 period. In the great river-deposit of the Wealden, the Bivalves 

 are forms proper to fresh water, belonging to the existing 

 River-mussels (Unio\ Cyrena and Cyclas ; but most of the 

 Cretaceous Lamellibranchs are marine. Some of the most 

 abundant and characteristic of these belong to the great family 

 of the Oysters (Ostreidcz). Amongst these are the genera 

 Gryphcea and Exogyra, both of which we have seen to occur 



