268 



HISTORICAL PALEONTOLOGY. 



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 Palceozoic 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 Terebraiida, Tere- 

 bratclla (fig. 194), Tereb?'atiilina, 

 Rhynchonella, and Crania (fig. 

 195). In the last mentioned 

 of these, 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- 



T93. — A small fragment of Esduirina 

 Oceani, of the natural size ; and a portion 

 of the same enlarged. Upper Greeusand. 



Fig. 194. — Terehratella Astieriana. Gault. 



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

 species — namely, Terebratulina striata — appears to be undis- 

 tinguishable from one now living — the Terebratulijia caput- 

 serpentis. 



Whilst the Lamp-shells are slowly declining, the Bivalves 

 {Lamellibrajichs) 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 i^Ostreidce). Amongst these are the genera 

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



