VARIATION IN TIME * 161 



that of the Gryphseas of the Lias as to make one 

 question whether it is really a prolongation of the 

 same branch. 



Another fine example of discontinuous series is 

 offered to us by the bivalve Molluscs of the family of 

 the Megalodontidce, which . have thick shells, tri- 

 angular in form, with flattened and slightly spiral 

 tops, and furnished with strong teeth in the 

 hinge. The earliest representatives grouped under 

 the generic name of Megalodon appear in the middle 

 Devonian of the Eifel under the form of the M . 

 cucullatus, with a relatively small and sub-rounded 

 shell. 



We must then go up to the top of the Trias 

 of the Eastern Alps to again meet with, in the 

 limestone of the Dachstein and the Hauptdolomit, 

 the Neomegaladontidce, now become gigantic and 

 modified in some of the details of their hinge. The 

 branch then prolongs itself in a more or less con- 

 tinuous manner through the Rhsetian and the Lias, 

 and after another gap we may see it terminate in 

 the Pachymegalodon of the upper Jurassic with a 

 very thick shell. 



In the group of Ammonites a learned specialist, 

 Gustave Sayn, has shown that the Pulchellia of 

 the Barremian stage, with its elegantly ornamented 

 shell, should be genetically attached to the smooth 

 and discoid Ammonites of the Valanginian, which 

 he designates by the name of Garnieria ; these in 

 turn are connected with the Oxynoticeras of the 

 upper Jurassic of Russia, and the latter might 

 perhaps be, notwithstanding the enormous geo- 

 logical lacuna which separates them, the direct 



M 



