ACADIC PEEIOD 521 



a wide distribution of the Asiatic faunas as far east as Appalachia and 

 Acadia. Beyond these lands there were at this time distinct Atlantic 

 faunas; in the Atlantic the Paradoxides faunas, and in the Pacific the 

 Olenoides and big-tailed trilobite biotas. The Saint Croix invasion 

 seems to have spread somewhat rapidly across the continent, particularly 

 in the north, where the continental seas also vanished earliest during the 

 Franconia emergence. In the southern parts of the United States the 

 marine sequence appears to be a longer and more perfect one, with less 

 clastic and more organic deposits. In the northern portion of the Missis- 

 sippian sea nearly all the strata are sandstones and shales. The writer is 

 not yet able to map these two events separately, and thus bring out more 

 clearly the cycle of sea movements. 



The Acadic of the Pacific realm of North America is marked by the 

 presence of the trilobite genera Kootenia, Zacanthoides, Bathyuriscus, 

 Asaphiscus, Neolenus, Olenoides, Dorypygella, Dorypyge, Damesella, and 

 Ogygopsis. Toward the top appear Crepicephalus texanus, Billing sella 

 coloradoensis (also in lower portion), and Eoorthis remnichia, these like- 

 wise passing into the western Upper Cambrian. No genus of brachiopods 

 having several common species is restricted to the Acadic, but in the 

 lower portion Micromitra pannula abounds, and in the higher, M. sculp- 

 iilis. According to Walcott (1908), the first appearance of Illcenurus 

 determines the base of the Upper Cambrian. 



The Acadic is the time of greatest abundance and differentiation of 

 inarticulate brachiopods. Throughout the world at this time there are 

 31 genera arid 243 species of brachiopods, while in the Georgic there are 

 20 genera and 75 species, with 23 genera and 137 species for the Upper 

 Cambrian (Walcott). During the latter time the inarticulates rapidly 

 drop out as stratigraphic factors, while the articulate calcareous-shelled 

 forms as rapidly differentiate and become some of the best time guides of 

 subsequent Paleozoic periods. 



In the upper Mississippi valley the Franconia horizon has Billing sella 

 coloradoensis, Agnostus joseplia, Chariocephalus whitfieldi, Conocepha- 

 lites diadematus, Lonchocephalus hamulus, and Ptychaspis. The Dres- 

 bach zone is marked by Lingulepis pinniformis, Dicellomus politus, and 

 Agraulos convexus. At Eau Claire, Wisconsin, the lower sandstones have 

 Agraulos woosteri and Conocephalites calymenoides. 



In the argillites at Braintree, Massachusetts, occur Paradoxides har- 

 lani, Ptychoparia rogersi, and Agraulos quadrangularis. 



The Atlantic fauna has recently been discovered by Edson in an intra- 

 formational conglomerate at Saint Albans, Vermont. Here Walcott 145 



14 « Perkins : Annual Report of the Geological Survey of Vermont, 1907-1908, p. 209. 



