SILURIC OR ONTARIO PERIOD 535 



Many of the Sihiric brachiopod genera of America also group them- 

 selves readily into two distinct realms — the Arctic and Atlantic. To the 

 Hudson sea are restricted the following genera : Capellinia, Orthotropia, 

 Dinooolus, Monomorella (both occur in the Saint Lawrence sea), Rhino- 

 bolus, and T rimer ella. It is very significant that all the platform- 

 bearing oboloids are of the Arctic realm or of northern Atlantic waters. 



In the three continental seas having Atlantic connections — the Saint 

 Lawrence, Appalachian, and Mississippian — there are many genera re- 

 stricted to these areas, most of which are also represented in Europe. 

 These are : Anabaia, AnastropJiia (may have representation in the Hud- 

 son sea), Atrypina, Delthyris, Dictyonella (may be in the Hudson sea), 

 Gypidula, Hindella, Homceospira, Hyattella, Mimulus, Orthostrophia, 

 Reticularia, Scenidium, Streptis, Stricklandinia (out of 16 species, only 

 2 in the Hudson sea), and Uncinulus of the stricMandi type. 



The trilobites are usually excellent indicators of provincial connections, 

 and as those of the American Siluric are practically all in harmony with 

 those of western Europe, it must be assumed that the Atlantic has been 

 the realm where these forms have developed. They are also good travel- 

 ers, for Van Ingen has identified three European species in Arkansas — 

 Acidaspis quinquespinosa, Deiplion foroesi, and Staurocephalus murclii- 

 soni. Further, of the 105 American Siluric species listed by Weller 

 (1907), at least 17 have a wide geographic range and occur in two prov- 

 inces. The trilobites as a rule, therefore, appear to have a long range in 

 time, certainly longer than the crinoids, but are not so persisting as the 

 brachiopods. 



During the time of the Clinton and early Rochester, a series of migrants 

 from the permanent basin of the Gulf of Mexico appear in the Mississip- 

 pian sea on the west side of the Cincinnati axis. Many of these trilobite 

 genera also occur in Europe, and later find their way into the province of 

 the Hudson sea. These are, according to Weller (1907) : Acidaspis, 

 Ampyx (restricted to the waters of the Gulf), Arctinurus, Ceratocephala, 

 Ceraurus, Corydoceplialus, Cypliaspis, Dalmanites, Deiplion, Dicranopel- 

 tis, Encrinunis, Metopoliclias, Odonto pleura, Phacops or Acaste, Proetus, 

 Splicerexoclius, and Staurocephalus. Other and northern Atlantic genera 

 are: Bronteus and Homalonotus. Still later migrants restricted to the 

 Hudson sea are: Harpes and Illcenus. In the Chicago area Weller has 

 described 12 species of the last named genus, and there are 2 other forms 

 from the same province. In other words, of the 17 American Siluric 

 species of Illamus. 1-1 are found in the area of the Hudson sea. 



The known trilobites of the American Siluric are. therefore, of the 

 Atlantic and Arctic realms. The great majority of genera, however, are 



XLVII — Bull. Geol. Soc. Am., Vol. 20, 1908 



