398 On American Geological History. 



The Palaeozoic Trilobites belong to the lower tribe of Crustacea, 

 and Crustacea rank low among Articulates. Moreover, Crustacea 

 (and the Articulata in general) did not reach their fullest develop- 

 ment until the Human Era. 



The Radiata were well represented in the Silurian periods ; but, 

 while inferior to the Mollusca as a sub-kingdom, only corals 

 and crinoids, the lower fixed or vegetative species, with rare 

 exceptions, occur in the Silurian of Molluscan Age. 



The Articulata and Radiata thus begin early, but with only the 

 lower forms in each, and neither is a leading class in any age. 



Viewing the history, then zoologically , the ages are, the Age 

 of AloUusks, of Fishes, of Reptiles, of Mammals, of Man. 



We may now change the point of view to the Vegetable King- 

 dom. The aQ^es thence indicated would be three : — 



I. The Age of Algce, or marine plants, corresponding to the 

 Silurian and Devonian. 



II. The Age of Acrogens, or flowerless trees, that is, the Lepi- 

 dodendra, Sigillariae, and Calamites, — corresponding to the Coal 

 Period and Permian ; a name first proposed by Brongniart, and 

 which may still be retained, as it is far from certain that the 

 Sigillarise and Calamites are most nearly related to the Coniferse. 



III. The Age of Angiosperms, or our common trees, like the 

 Oak, Elm, &c., beginning with the Tertiary. 



The interval between the second and third of these ages is 

 occupied mainly by Coniferje, the Pine tribe, and CycadejB, the 

 true Gymnosperms, species of which were abundant in the Coal 

 Period, and have continued common ever since. The Coniferse, 

 in the simplicity of their flowers and their naked seed, are next 

 akin to the Acrogens or flowerless trees. Although in the main 

 a flowerless vegetation, for the few supposed remains of flowers 

 observed abroad have been recently referred to undeveloped 

 leaf-buds, it appears probable from the observations of Dr. New- 

 berry, that there were some true flov/ers over the Ohio prairies, — 

 apparently monocotyledonous, and related to the Lily tribe. But 

 no traces of Palms or monocotyledonous trees have been found 

 in the coal fields of this country. 



Combining: the results from the animal and veofetable kinof- 

 doms, we should introduce the Age of Acrogens, for the Coal 

 Period and Permian, between the Age of Fishes and Age of 



