FISHES ABUNDANT. 



39 



velopment of the humblest vertebrate class Fishes. There 

 is here, as there was in the Silurians, an abundance of zoophytes, 

 corallines, crinoids, crustaceans, and mollusks, but mostly pre- 

 senting those inferior variations which naturalists regard as 

 constituting distinct species ; speaking strictly, 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 gradually 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 gradually 

 filling up, or the intrusion of a new mineral infusion 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 ex- 

 tensive changes of species occur without any apparent depend- 

 ence on at least some of these causes ; so that, in these instances, 

 some other explanation remains to be sought for. 



Corals (favosites, cyathophylla, stromatopora) are amongst 

 those genera which pass from the Silurians to the Devonians ; 



FIG. 15. 



FIG. 16. 





Favosites polymorpha. Cyatkophyllum quadrigeminum. 



they are so abundant, as in some places to constitute entire 

 strata (Devonshire marbles). The crinoids and trilobites are 



