36 DEVONIAN ERA. 



out of about eight hundred so-called species of the Silurian 

 epoch, one hundred pass into the Devonian formation, where, 

 however, they gradually disappear, while new ones as gradu- 

 ally take their place. For such changes of species, adopting 

 this word in the sense usually attached to it, geologists suggest 

 causes in physical changes, as the rise of a sea-bottom by 

 gradual filling up, or the intrusion of a new mineral material 

 into the ocean, or one of a more decisive kind proceeding 

 from such revolutions as are indicated by unconformableness 

 in the strata. But on this point much obscurity at present 

 rests, for, as our survey is extended into other countries, it is 

 found that extensive changes of species occur without any 

 apparent dependence on at least some of these causes ; so that, 

 in these instances, some other explanation remains to be 

 sought for. 



Corallines (favosites, cyathophylla, stromatopora) are 

 amongst those genera which pass from the Silurians to the 

 Devonians ; they are so abundant, as in some places to con- 

 stitute entire strata, (Devonshire marbles.) The crinoids and 

 trilobites are also continued as families throughout this era. Of 

 the latter we have a new species (brontes), marked by several 

 new features, including a set of claws resembling those of the 

 common lobster, and the whole length of which is judged to 

 have been not less than four feet. Some of the new brachio- 

 pods are of very peculiar shape ; amongst the gasteropods are 

 some which approach existing forms. The lordly cephalo- 

 poda continue to be largely represented, but in a considerable 

 change of form ; for while the chief animals of this class in the 

 Silurians (orthoceratites) had a simple, straight, or slightly 

 curved shell, those new to the present era (as clymenia) had 

 one forming a complete spiral. 



The most remarkable circumstance connected with the 

 Devonian formation, is its presenting us with fish. A few 

 faint traces of this class had, as we have seen, been presented 

 in the Upper Silurians of our own country, though, it may be 

 remarked, wanting in the corresponding rocks of Russia. 

 We are now to see such memorials of them in the Devonian 

 formation as show that the seas of that era had in many 



