CRETACEOUS ERA. 



147 



and tapering tower -like (turrilites), or in some other 

 grotesque and simulative forms. This flush of generic 



.1, Ancyloceras ; 2, Scaphites ; 3, Crioceras; 4, Hamites; 5, Turrilites. 



type, and that on the eve of their decline, has given 

 rise to many hypotheses ; and by those who associate mo- 

 dification of form with the influence of physical conditions, 

 obnoxious changes in the waters of deposit are supposed 

 to have been the proximate causes of these curious and 

 sportive shapes. It is true that an influx of fresh water 

 into a marine area, or vice versa, is often attended by ciiri- 

 ous changes in the indwelling mollusca, and that new con- 

 ditions of cultivation produce strange sports among the 

 varieties of the gardener but the forms of these cretaceous 

 shells is too decisive and persistent to be otherwise ex- 

 plained than by the introduction of new genera, in obedi- 

 ence to some great but unknown law of creation. The 

 fish life of the chalk period presents us with many of the 

 old placoids and ganoids (the sharks, rays, and sauroids) of 



