214 ^^^^ American Geologist. October, looi. 
PALEONTOLOGICAL SPECULATIONS. 
II. 
Biological Crises. 
By L. P. Gratacap. 
(Continued from Vol. XXVII. p. 75.) 
Surveying a paleozoic series of fossils the student is im- 
pressed with the recurrence of periods of great fecundity or, 
more exactly, for it does not have reference to mere individ- 
ual multiplication, periods of especial creativeness. A hori- 
zon is encountered when the rocks suddenly ( ?) swarm with 
related forms of life, quite manifoldly developed. They rep- 
resent those moments, in the survey of a cabinet, when the 
visitor finds the sort of animal life (presented as fossils) ex- 
panding over a number of shelves or even cases, which, in 
related forms, are elsewhere found limited to one or a few. 
Such are for instance, the numerous specimens of trilo- 
bites in the Upper Cambrian, the Cystidea in the Upper 
Silurian, the laincllibraiichs in the Devonian, the crinoids in 
the Lower Carboniferous. It would be preposterous to call 
these ocurrences unannounced, but they have some of the 
character of surprises.* Dr. Hall (Preliminary Notice of the 
Fauna of the Potsdam Sandstone) in speaking of the re- 
mains of primordial trilobites in Wisconsin and Minnesota 
cClludes to their great numbers ; "the material consists of 
glabellae, separated cheeks, caudal shields, and fragments 
of thoracic articulations either lying separately or crowded 
together, sometimes forming the principal part of layers one 
or two inches or more in thickness." 
Developed with these were the great numbers of inartic- 
ulate brachiopods, so that in individual enumeration, as Owen 
has remarked, the "number is immense ; some slabs so cov- 
ered with shells, that it would be difficult to place the finger 
on a spot without touching some of them," and in less than 
500 feet, a recurrence of six trilobite beds. 
The variation and wealth of specific forms is well indi- 
cated in Whitfield's remarks on his own species Crepiceph- 
•Lapparent has indeed used tbe expression, in speaking of this primordial 
fauna, "I'eclosion, qui semble presque subite, de la remarquable faune de crus- 
taces a laquclle Barrande a donne le nom de faune primordiale." 
