196 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



sion. The forms of the larger sizes of this branch are 

 found in the terminal strata of the Aquitanian, for 

 instance, in the Vicentine region. In the great class 

 of the Echinida, the genus Clypeaster commences 

 by forms of mediocre size in the Eocene of Upper 

 Italy and Egypt, and attains in the Miocene the 

 considerable dimensions of the Clypeaster altus, 

 crassicostatus and ^gyptiacus of Malta, the Vienna 

 Basin, and the environs of Gizeh. Similarly, 

 the small Megalodon of the Devonian announce 

 and precede the enormous Triassic shells of this 

 genus. The shells of the tribe of Diceratinae which, 

 with the genera Diceras and Heterodiceras, peopled 

 the coral reefs of the higher Jurassic, attain in 

 the Tithonic reef of the Echaillon the astounding 

 dimensions of certain samples of the Diceras Luci, 

 which form part of the collections of the University 

 of Grenoble. We may even draw an argument from 

 these enormous Dicerata to affirm that this branch 

 cannot have had any direct descendants in the 

 Cretacean. The Ammonites, notwithstanding the 

 unfavourable conditions pointed out above, show 

 us, in the terminal mutations of some branches, 

 species of very great size; for instance, the Pina- 

 coceras of the Trias, the Arietites of the Lias, the 

 Stephanoceras of the Portlandian, the Ancyloceras 

 of the Aptian, the Scaphites of the Senonian, the 

 Pachydiscus of the terminal strata of the Upper 

 Chalk ; and it is with this last genus, composed 

 of forms of large size and senile characteristics, that 

 the evolutionary activity of the great group of the 

 Ammonites appears definitely to exhaust itself. 

 Finally, I shall further point out in the great class 



