FOSSIL LAMP-SHELLS. 



227 



tures, which are useful to the palaeontologist in 

 enabling him to classify these fossils. In many 

 genera there is a limy, brittle, and delicate structure, 

 assuming either the shape of "plates," "loops," or 

 " spirals." Very frequently we get fossils in which 



Fig. 202. — Orthis striatitla 

 (Devonian), 



Fig. 203. — Interior of 6)*/r^r, 

 showing spiral coils. 



these are still preserved, and the experienced worker 

 among the limestones of the Palaeozoic rocks knows 

 that the " chert-bands " — that is, the accumulation of 



I'lg. 204. — Spirifcr trigon- 



alis, showing internal 



coil where portion of shell 



is removed (Carboniferous 



limestone). 



Fig. 2o$.—Spiri/er spedosus 

 (Devonian). 



chemically combined silica and lime (analogous to 

 the flint-bands and nodules of the Chalk) — are the 

 best places in which to look for the internal spirals, 

 loops, plates, etc., of the fossil Brachiopods. For in 

 such places the outer shell has been dissolved away, 

 and the internal and more delicate " skeleton " has 



