52-4 C. SCHUCHERT PALEOGEOGRAPHY OF XORTH AMERICA 



sippian sea was finally almost entirely withdrawn, for there is an erosion 

 interval above the formations named. The seas retreated southward. 



In a general way it may be said that the Ozarkic period begins with the 

 trilobite genus Dikelocephalus and the first distinct molluscan fauna. 

 THrich has collected about 200 species of this period, nearly all of which 

 are new and await description. The trilobites and inarticulate brachio- 

 pods (greatly reduced in species) are still Cambrian in aspect, while 

 the new faunal feature consists in a rapid evolution, in form and size, 

 of the coiled gastropods and of both straight and coiled cephalopods. 

 The latter are distinguished from those of subsequent periods by the ex- 

 ceedingly close arrangement m of the septa. Xo bivalves are here repre- 

 sented, no bryozoa, graptolites (Cladophora are present), crinoids, star- 

 fishes, nor ostracods (all the so-called forms are determined by Ulrich to 

 be phyllopods). During the Ozarkic period, for the first time in the his- 

 tory of the earth, the animals living in the sea, especially the mollusks, 

 began a general secretion of calcareous skeletons. Of course, the sponge- 

 like corals of the Georgic had the "lime habit" much earlier, but until 

 the Ozarkic in none of the earliest faunas did the secretion of lime be- 

 come a factor among many types of invertebrate animals. That this 

 mode of protection was of great benefit to the creatures possessing it, is- 

 seen not only in the rapid rise of genera and species in the Ozarkic, but 

 also in the marked increase in the size of individuals. This evolution is 

 particularly noticeable in the middle and upper beds of this period. 



In the lower part of the Ozarkic system of Minnesota and Wisconsin 

 occur Agnostus disparilis, A. parilis, Illcenurus quadratus, Dikelocephalus 

 minnesotensis. D. pepinensis, Aglaspis barrandi (the oldest limuloid),. 

 Owenella antiquata. Holopea sweeti, Ophileta, Baphistoma, etcetera. 



The conglomerates of the "Quebec series" also bear this fauna, but in 

 the Saint Lawrence trough the actual formations are unknown. These 

 species have been assembled by Walcott. 147 This fauna chiefly brings to 

 mind those from near Saratoga Springs, Xew York, where Walcott col- 

 lected Dikelocephalus hartti, D. speciosus, Agraulos saratogensis, Lingu- 

 lepis acuminata, etcetera. 



Ordovicic Period of Authors 



The Ordovician is usually regarded as a period shorter in duration than 

 the "Cambrian," with a complexity of faunas not more varied nor difficult 

 of interpretation than those of the Siluric. Accompanying this memoir 

 are nine maps covering this time ; at least five additional ones will have to 



" 7 Walcott: Bull. U. S. Geological Survey, no. SI, 1891, p. 151. 



