308 /. D. Dana on American Geological History. 



and crinoids, the lower 'fixed or vegetative species, with rare ex- 

 ceptions, occur in the Silurian or 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._ 



Yiewing the history, then, zoologically, the ages are — the Age 

 of Mollusks, of Fishes, of Reptiles, of Mammals, of Man. 



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

 dom. The ages 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, Sigillarige, 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 

 Sigillariae 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 Coniferse, the Pine tribe, and Cycadese, 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 seeds, are next 

 akin to the Acrogens or flowerles 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 flowers 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 vegetable king- 

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

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

 Reptiles, — a space in time zoologically occupied by the overlap- 

 ping of these two ages.* 



* This Age, would perhaps be more correctly styled, the Age of Conifers, as 

 Conifers, a higher group than Acrogens, were among the earliest of all land plants, 

 occurring in the upper Devonian as well as Carboniferous ; and the ages in other 

 cases are named from the superior group of species. Yet as the Acrogens were es- 

 pecially characteristic of the era, and the Conifers have their fullest development in 

 the present age, the name above given seems to be preferable : — unless it prove true 

 that the Sigillariae and Calamites are actually related to the Coniferse as urged 

 by Brongniart. Zoologically, the age has some title to the name, Age of Amphib- 

 ians. But before it closed, true reptiles had appeared. It is a significant fact that 

 the Amphibians in some cases appear to have approached true reptiles, as much as 

 some of the genera of Acrogens the Conifers. An interesting example of this, from 

 the coal formation of Ohio, has recently been mentioned by Dr. J. Wyman, (Tenth 

 Meeting Amer. Assoc. at Albany). 



