THE SILURIAN PERIOD. 409 



period, Silurian fishes were somewhat abundant and will be considered 

 in connection with the closing Silurian fauna. 



Marine plants. — Knowledge of marine plant life remains, as before, 

 very unsatisfactory. While theoretically it must have been abundant, 

 only obscure markings of the fucoidal type and a few evidences of 

 higher forms have been found, and the interpretation of these is more 

 or less doubtful 



Foreign Faunas and Migratory Relations. 



Deferring the consideration of the peculiar closing fauna for a moment, 

 the foreign relations of this rich and varied Mid-Silurian assemblage 

 claim attention. The general progress of life on other continents, so 

 far as known, was similar to that on the American, but perhaps less 

 pronounced and symmetrical. A part of this similarity of progress 

 was doubtless due to parallel evolution under similar general condi- 

 tions, whi!e a part was quite certainly due to intermigration Where 

 the forms on different continents were merely similar, it is at present 

 an open question whether the similarity was due to intermigration 

 with modification or to independent evolution along similar lines; but 

 where the forms on separate continents are specifically identical, espe- 

 cially if the species be peculiar and aberrant, it may safely 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 Goniophyllum pyramidale, already mentioned (Fig. 188, d), which 

 is found with identical idiosyncrasies in the island of Gothland in the 

 Baltic Sea, and in Iowa. This evidence of migration is much strength- 

 ened by the presence in Gothland of three peculiar genera of crinoids 

 that are also found in the upper Mississippi region. Besides such 

 special cases, many prominent and familiar species were common to 

 America and Europe, among which, neglecting the wide-ranging grapto- 

 lites, were Favosites gothlandica, Halysites catenulatus, Heliolites inter- 

 stinctus, Dalmanella elegantula, Pentamerus oblongus, Spirifer radiatus, 

 Rhynchotreta cuneata, Atrypa reticularis, Orthoceras annulatum, Homa- 

 lonotus delphinocephalus, and others. Some of these are also found 

 in Asia, Australia, and New Zealand. Silurian genera occur in Bolivia, 

 Peru, and Brazil, some species of which resemble North American and 

 European species, but they have not been proved to be identical. Migra- 

 tory connections in this case cannot be affirmed, but such connections 



