FOSSIL LAMP-SHELLS. 



235 



adorning it. The noble Pentamems Knightii of the 

 Aymestry limestone used to be so abundant that it 

 was used to mend the roads with. Terebratula sella 

 is no less abundant in the Oolite ; T. carnea and T. 

 globosa, equally so in the White Chalk of England, in 

 almost every Chalk-pit. T. grandis, of the Suffolk 

 White Coralline Crag,, is the hugest and finest of all, 

 but uncommon enough for the young geologist to be 



Fig. 217. Diagram showing the various stages of development in a recent 

 Brachiopod, from b, c, larval stages, etc,, to a, adult animal. 



delighted when he finds a specimen with both valves 

 still united. 



The fossil Brachiopoda have undergone great 

 changes. In the Cambrian, Silurian, and Devonian 

 slates we find them chiefly as casts, the fossils being 

 natural casts of the interior of the shells, and the 

 slates retaining the impressions of the external ribs 

 and other markings. On the top of Snowdon, and 

 elsewhere, we find them imbedded in volcanic ashes. 



