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 siriatus : a, exterior of shell ; i, interior, showing spiral coils ; 

 r, 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 

 TerebraUilce) occupies more than half of the interior 



Fig. 207. — Rhynchonella Fig. 208. — Terehra 

 //c'urodoH (Carhoniferov 



limestone). 



it(/a hastata (Car- 

 boniferous limestone). 



Fig. ■2og.—Sptri/er cuspidaius 

 (Carboniferous limestone). 



of the shell. In the Spiriferce 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 Pentamenis we have it developed as a series 

 o^ plates J dividing the interior into five parts, like 



