DISTRIBUTION 97 



evidence of identity and close relationship in other faunas, have led to the 

 opinion especially developed by Weller, 1 and afterwards supported by Cham- 

 berlin and Salisbury, 2 that there was a migratory connection between the 

 Silurian faunas of northern Europe and the interior of America. 



Of the 13 Carboniferous genera, 9 are common to both continents; all but 

 one are represented in America, while 3 American genera are not found in 

 Europe. The recent addition of the two specialized forms, Amphicrinus and 

 Onychocrinus, to the list of genera common to both areas greatly emphasizes 

 the resemblance of the two faunas, and accords with the opinion held by the 

 later authorities in Paleogeography that in this age also, as in the Silurian, 

 there was a migratory communication between the American and European 

 continents by one of the northern routes. 3 



Of the 9 Devonian genera, 3 of which begin in the Silurian and 4 pass 

 into the Carboniferous, 8 occur both in Europe and America; but there is little 

 similarity in the species from the two continents, which is to a large extent true 

 for the other orders of crinoids. 



The development of the crinoids as a whole was greater in the Lower 

 Carboniferous than in any other Paleozoic period. The physical conditions of 

 the American continent during that epoch, with its extensive shallow seas and 

 general prevalence of clear waters, were peculiarly favorable to the expansion 

 of the crinoidal faunas, and this vast basin became so densely populated with 

 these organisms that their calcareous remains constitute the greater part of the 

 sedimentary deposits. The Mississippi valley was the center of this develop- 

 ment, and it extended westward — its resulting strata now underlying the great 

 plains area — into the Rocky Mountain region, where abundant evidence of its 

 existence, with more or less migrational changes of species, is to be found, from 

 New Mexico to Montana, and westward as far as Nevada. 



The rise, expansion and decline of the Flexibilia are in substantial ac- 

 cordance with those of the Paleozoic crinoids in general, viz., rapid develop- 

 ment in the Silurian, recession in the Devonian, attainment of the acme in the 

 Lower Carboniferous, followed by extinction near or shortly after the close of 

 the Paleozoic. 



The hitherto prevailing idea that the Crinoidea, as represented by their 

 surviving order, the Articulata, are a dwindling race almost on the verge of 

 extinction, must now be modified, in view of the great extension of knowledge 

 of the Recent crinoids during the past few years, largely through the medium 

 of the researches of Mr. Clark in the National Museum before alluded to upon 

 American and foreign collections. Instead of only 12 genera and 212 species 



1 Journal of Geology, vol. 4, 1896, pp. 166-173 ; ibid., vol. 6, 1898, pp. 692-703. 



2 Chamberlin and Salisbury, Geology, vol. 2, 1906, p. 410. 



3 Ibid., p. 522. 



