CRETACEOUS ANIMALS. 



481 



spirally, like a Gasteropod, as in Turrulites and Helioceras. Belem- 

 nites (Fig. 796) also continue, though in diminishing numbers. 



These strange forms have been likened by Agassiz to death-contor- 

 tions of the Ammonite family ; and such they really seem to be. From 



Figs. 



Fig. 794. 



-795.— Cretaceous Gasteropods: 793. Cypraea Matthewsonii (after Gabb)>. 794. 

 rb.ais falciformis (after Gabb). 795. Scalaria Silliniani (after Lesquereux)i 



Apor- 



the point of view of evolution, it is natural to suppose that under the 

 gradually-changing conditions which evidently prevailed in Cretaceous 

 times, this vigorous Mesozoic type would be compelled to assume a 

 great variety of forms, in the vain attempt to adapt itself to the new 

 environment, and thus to escape its inevitable destiny. The curve of 

 its rise, culmination, and decline, reached its highest point just before 



Fig. 796. — Belemnites impressus (after Gabb) 



it was destroyed. The wave of its evolution crested and broke into 

 strange forms at the moment of its dissolution. 



Among Crustaceans, the Brachyurans, short-tailed Crustaceans 

 (crabs), which were barely introduced in the Jurassic, are here repre- 

 sented by several genera. 



31 



