POTSDAM PERIOD. 203 



higher group. The Phyllopods are other Entomostracans, and 

 in some respects they constitute a group of high grade, having 

 affinities with the Macrourans as shown in the general form of the 

 species. The Trilobites and Phyllopods are examples of compre- 

 hensive types [synthetic of Agassiz), — types comprehending, along 

 with their own characteristics, some of those of other tribes which 

 were yet uncreated, but which were to exist in the future unfold- 

 ing of the system of life. 



As far as has been deciphered in the history of the Primordial 

 period, there was no green herbage over the exposed hills ; and no 

 sounds were in the air save those of lifeless nature, — the moving 

 waters, the tempest, and the earthquake. 



(b.) Exterminations. — The life of the Potsdam period changed 

 much during its course, and at one time — the close of the first 

 epoch — there was nearly a complete extermination of the species, 

 requiring a repeopling of the seas for the succeeding epoch. Two 

 or three species, including a Lingula, exist, but the others do not 

 reappear. Among the Trilobites, the genus Paradoxides, some of 

 whose species were the largest of known Crustaceans, became en- 

 tirely extinct ; most of the other genera remained, but were repre- 

 sented by new species. 



At the end of the Calciferous epoch there was a second exter- 

 mination, obliterating wholly the life of the Primordial period. 

 Some species had disappeared before in the progress of the epoch, 

 for these destructions were not confined to the grander transitions 

 in the strata ; and there had also been new additions to the species 

 at intervals, judging from the successions in the Quebec rocks. 

 These exterminations and creations in the progress of a period, and 

 more general exterminations and creations at the end of an epoch 

 or period, were a common feature throughout the earth's geolo- 

 gical progress. 



5. Reality of the Primordial or Potsdam period in America, and 

 its equivalency with the European. — The fact that the Potsdam 

 and Calciferous epochs constitute together one period in the history 

 is shown by the transitions of the strata, and more especially by 

 the resemblances between the two in living species. This has 

 become more apparent since the recent discoveries in the Canadian 

 geological survey under Sir William Logan. The types of life of a 

 period are of two classes : those of one class are characteristic of 

 the period, and have their fullest exhibition in its course ; the others 

 look onward to a fuller expression in some part of future time. 

 The genera of Gasteropods and Cephalopods are of the latter kind ; 

 for Pleurotomaria, Maclurea, Orthoceras, and other genera of these tribes 



