228 OUR COMMON BRITISH FOSSILS. 



been preserved, and is now perhaps encrusted with 

 microscopical crystals of silica. 



In all cases, this internal loop, spiral, or whatever 

 shape it may have assumed, is merely the mechanical 



Fig. 206. Spirifer strtatus : a, exterior of shell ; b, interior, showing spiral coils ; 

 c, portion of coil (Carboniferous limestone). 



support of the brachial or labial coils, about which 

 remarks have already been made. 



Not unfrequently this internal loop (as in some 

 Terebratulcz) occupies more than half of the interior 



Fig. 207. Rhynchonella Fig. 208. Terebra- 

 pleurodon (Carboniferous tula hastata (Car- Fig. 209. Spirifer cuspidatus 

 limestone). boniferous limestone). (Carboniferous limestone). 



of the shell. In the Spirifera there are two conical 

 coils or spires (whence the name of the genus 

 " spire-bearing "), the apices of which are on each side. 

 In the Pentamerus we have it developed as a series 

 of plates , dividing the interior into five parts, like 



