FIKST FORMS OF LIFE. 



85 



animals, as the nautilus, cuttle-fish, and poulp. The cephalo- 

 poda are, among invertebrate animals, what the feline and 



FIG. 11. 



FIG. 12. 



Sepia officinalis, or Common 

 Cuttle-fish. 



Shell of Nautilus pompilius, cut open 

 to shotv the chambers and the siphon. 



FIG. 13. 



musteline animals are among the vertebrates, or the falcons 

 among birds. They pursue a free-swimming life, with an 

 organization powerfully adapted for making prey of the 

 humbler marine animals. Na- 

 turalists regard them as having 

 been eminently the police of 

 those early ages, keeping down 

 the teeming population of the 

 Silurian seas. The genera which 

 appear are, however, not the 

 most exalted of their order, al- 

 though many were of great size. 

 Nor are they so abundant in the 

 Lower Silurian period, either in 

 species or individuals, as they 

 afterwards became. 



Such in outline is the fauna 

 of the Protozoic Period, or that 



D2 



Lituites cornu-arietis, a Lower 

 Silurian Cephalopod. 



