230 



OUR COMMON BRITISH FOSSILS. 



largely developed, and assumes such a variety of 

 forms that it proves useful for classificatory purposes. 

 The spiral coils are found in Spirifer, Spiriferina, 

 Athyris, Retzia, Merista, etc. In the Rhynchonellidce 



Fig. 213. Orthis resupinata (Car- 

 boniferous limestone). 



Fig. 214. Productus scabriculus 

 (Carboniferous limestone"). 



the fleshy arms are usually supported by the pair of 

 short plates alluded to, and sometimes by spirally 

 coiled ribbons, closely pressed together, and having 



the apices of the flattened 

 spires meeting within, in- 

 stead of the reverse way, as 

 in the Sfirifera. This struc- 

 ture is best and most com- 

 monly found in A trypa. 



The enormous size of 

 the plates inside the Pen- 

 tamerus is finely seen in the large and beautiful Upper 

 Silurian fossil Pentamerus Knightii, which frequently 

 splits open with great ease along the planes or faces 

 of these internal divisions. 



The Cranias are easily identified, for they are 



Fig. 215. Productus horridus 

 (Permian, limestone). * 



