2 7 6 



HISTORICAL PALAEONTOLOGY. 



of the Eschar idee, the genera Eschar a and Escharina (fig. 193) 

 being particularly well represented. Most of the Cretaceous 

 Polyzoans are of small size, but some attain considerable di- 

 mensions, and many simulate Corals in their general form and 

 appearance. 



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 Terebratula, Tcre- 

 bratella (fig. 194), Terebratulina, 



Rhynchonella, and Crania (fig. 

 193. A small fragment of Escharina 



Oceani. of the natural size; and a portion 195). I n the last mentioned 

 of the same enlarged. Upper Greensand. Q these> the shdl Js 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- 

 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 Terebratulina caput- 

 serpentis. 



Whilst the Lamp-shells are slowly declining, the Bivalves 

 (Lamellibranchs) are greatly developed, and are amongst the 



Fig. 194. Terebratella Astieriana. Gault. 



most abundant and characteristic fossils of the Cretaceous 

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



