THE SILURIAN PERIOD 555 



perhaps less pronounced and symmetrical. Where the forms on 

 different continents were merely similar, it is at present an open 

 question whether the similarity was due to intermigration, or to 

 independent evolution along similar lines; but where the forms on 

 separate continents are identical, especially if the species are peculiar 

 and aberrant, 'it may be assumed that they had a common origin, 

 and that migration is indicated. A striking case of this kind is 

 presented by the quadrangular operculated coral already mentioned 

 (Fig. 403, d), which is found with identical idiosyncrasies in the 

 island of Gotland in the Baltic Sea, and in Iowa. The evidence of 

 migration is strengthened by the presence in Gotland of three or 

 more peculiar genera of crinoids that are found also in the upper 

 Mississippi basin. Besides such special cases, many prominent and 

 familiar species were common to America and Europe, and some of 

 them are also found in Asia, Australia, and New Zealand. Migra- 

 tory connections between North America and these lands may 

 therefore be assumed. Since thirty or more species are known to be 

 common to North America and Europe (the interior of North Amer- 

 ica and Sweden and Great Britain), and since these embrace a wide 

 range of genera of very different habits, there is a strong presumption 

 of migration between North America and northwestern Europe. 



It has already been stated that the interior sea of North America 

 seems to have had no free communication with the sea to the east 

 or south, and that its extension far to the west is doubtful. To 

 the northward, on the other hand, the series of remnants of Silurian 

 formations, probably once connected, point to a broad thoroughfare 

 for shallow-water life between North America and Europe by way 

 of Greenland. 



The faunas that occupied the Appalachian trough, the St. Law- 

 rence embayment, and the Atlantic coast appear to have had 

 but limited connection with the fauna of the interior during the 

 mid-Silurian epoch. In the present state of knowledge, the faunas 

 of the Appalachian trough bear the aspect of provincialism. 



The Closing Restrictional Stage 



Following the luxuriant life of the mid-Silurian epoch, there 

 came, in North America at least, a notable decline, due to the 



