146 Cretaceous. 



The relation'between the invertebrate kingdom of the Cretaceous 

 period and the living invertebrates is shown (according to present 

 identifications) by the survival of more than one third of the Creta- 

 ceous genera, though all Cretaceous species have become extinct. The 

 survival, however, in different classes, is by no means uniform. In the 

 class Polypi, of sixteen Cretaceous genera, six are living. In the 

 class Echinodermata, of twent3'-two genera, eight are living. In the 

 class Bryozoa, of thirt^^-two genera, nine are living. In the class 

 Brachiopoda, of six genera, five are living. In the class Gasteropoda, 

 of one hundred and seventy-four genera, ninet3^-six are living. In the 

 class Lamellibranchiata, of one hundred and sixty-four genera, seventy 

 are living. But in the class Cephalopoda, where there were more than 

 thirty genera and subgenera, all have become extinct except a single 

 genus, the Nautilus. 



The connection between the vertebrates of the Cretaceous period, 

 and the liA'ing vertebrates, is, seemingly, much farther removed. No 

 Cretaceous genera of birds or mammals survive. In the class Reptilia, 

 where more than seventy-five Cretaceous genera have been determined, 

 only three genera are known to have survived, Crocodilus, Trionyx and 

 Emys. A few species of fishes, found in the Cretaceous, have been referr- 

 ed to living genera, and probably some of them are correctly so referred; 

 but from the great difl'erentiation observed in the vertebrates, during 

 the long period of time which has transpired, we can not expect to find 

 many forms preserving unchanged their ancient outlines, though we may 

 be able to trace backward the living genera into what we call distinct 

 ancestral genera or families. 



This closes our remarks upon the Mesozoic period, and we will now 

 take up the Caenozoic. There is no great break in animal or vegetable 

 life in passing from the Mesozoic to the Ctenozoic, as early geologists, 

 from very limited observations, supposed. Indeed, it maybe said- to 

 be a most propable hypothesis that there are no breaks in genealog- 

 ical trees. All organic life has descended from ancestral forms, and' 

 among the vertebrates, in the later geological periods, profitable accre- 

 tions or accessions of important parts or functions have been developed 

 in successive generations. This will become more apparent as we pass 

 from one group of rocks to another in the Tertiary period. 



