TINY ROCK-BUILDERS. 151 



along with others, are found in the Devonian rocks 

 about Plymouth. Another very common Devonian 

 coral is Favosites cervicornis (Fig. 39). 



The Devonian corals are difficult to hammer out, 

 as their interstices are filled in with hardened 

 sediment, so that the student will want to cut and 

 polish slices in order to examine their structure and 

 determine their species. Their extreme beauty, 



FIG. 39. Section of Favorites cervioornis. 



however, will repay the trouble or the cost of pro- 

 curing polished slabs and microscopical sections. 



The climax of coral development was reached in 

 the earlier Carboniferous ages. From the limestones 

 of this system over a hundred and forty species have 

 been catalogued. And yet, as Mr. Etheridge has 

 pointed out, of fifty species from the mid-Devonian 

 rocks none have passed to the Carboniferous, so 

 complete are the distinctions between corals de- 

 posited under different conditions. One of the most 

 frequent of the Carboniferous fossils is Lithostrotion, 



