THE AGE OF INVERTEBRATES OF THE SEA. 55 



North America, and Australia. It is curious, too, that while 

 [he Graptolites thus mark a definite geological time, and seem 

 to disappear abruptly and without apparent cause, they are the 



I 







a. 



Fig. 37. — Silurian Grapt.litida. 

 a, Grapiolithus. b, Diplogmpsm. c. Phyllo^rapsus. d, Tetragrapsus. 



e, Dtdymograpsus. 



first link in the long chain of the Hydroids, which, though 

 under different family forms, continue to this day, apparently 

 neither better nor worse than their perished Palaeozoic relatives. 



F,0.38^Central portion of Grap^-;lite, F.o. 39--^^ ^£^'^^^ -M^ ^^V- 



with membrane, or fljat (Dic/w- •*"•"'■ *- 



grapsus octobrachiatus, Hall). 



There is a group of little Stony Corals {Chaetetes\ which were 

 possibly also the cells of Hydroids, that have a similar history. 

 They are the only known Corals that date so far back as the 



