GEOLOGICAL BIOLOGY. 



in these lists the life-period of most of the species is for the 

 length of a single formation. 



Each of the Chief Types Represented at the Initial Period of 

 the Genus. It will be noticed also that each of the four prin- 

 cipal groups had representatives in the Niagara epoch, and 

 only a single species of Spirifer is reported from a Lower 

 horizon. The other two groups, Ostiolati and Glabrati, did 

 not appear till the Devonian, but these both appeared together 

 in the Upper Helderberg. 



Three Epochs of Special Expansion with Slow and Gradual 

 Change During the Rest of the History of the Genus, Of the 20 



Silurian Devonian Carboniferou 



VI. GLABRATI 



FIG 



neric groups, according to the 



. 07. Table representing the expansion of the Spirifers in subgeneri 

 classification noted by Hall, and elaborated on page 313. 



races into which the known American species are subdivided, 

 7 are reported from the Niagara; I begins in the Lower Hel- 

 derberg, 3, Oriskany, 4, Upper Helderberg, I, Hamilton, and 

 4 at the base of the Carboniferous, i.e., over a third of the 

 known races began at the first fauna in which the genus ap- 

 pears in North America (except the one species in the Clin- 

 ton); 7 more began near the base of the Devonian, and 4 

 began at the opening of the Carboniferous. This special 

 rapidity of appearance of new types at the three periods 

 marking the beginnings of three geological systems points at 



