RAYMOND: CORRELATIOX OF THE ORDOVICIAX STRATA. 269 



Scandinavia, biit that in turn though it received certain contributions 

 from the late Cambrian faunas of the region in which it developed, 

 owed much of its richness to types developed further south in the 

 Tremadoc of Bohemia, France, and England. Ha^■ing once gotten a 

 foothold in northern Europe, the fauna developed very rapidly there, 

 but apparently in an enclosed basin, for this fauna, as a whole, is 

 unknown outside Scandinavia and Russia. Here, however, the factor 

 of bottom control must be taken into account. As we have shown 

 (p. 222), it is generally recognized that the black shales with the 

 Didymograptus-Tetragraptus-Phyllograptus fauna were deposited 

 at the same time as the "Orthoceras limestone," and in the same 

 sea, but under different physical conditions. As is well known, 

 the graptolite faunas did migrate, and very widely, but they did not 

 carry the bottom fauna with them. If we adopt the rather generally 

 accepted opinion that the graptolites were pelagic animals, supported 

 either by floats or by their attachment to floating bodies such as sea 

 weeds, we may conceive that the graptolites may have been distrib- 

 uted within a very short time, by the power of ocean currents, over 

 very wide areas, while the influence of a strong current or of a cold 

 current, impinging upon headlands, or the presence of vast expanses 

 of sandy or muddy bottoms, may have long delayed the migration of 

 bottom-living animals. We seem to have an excellent example of 

 this in the case of Shumardia and certain associated species of the 

 Ceratopyge limestone. In Sweden and Norway, Shumardia is rather 

 abundant in the shale and limestone making up the Ceratopyge zone, 

 and this zone is above the shale with Didyoncma flahcJUformc, but 

 below that of Tetragraptus and Phyllograptus. The Shumardia 

 limestone of America, however, (at Point Levis) is very high in the 

 Tetragraptus-Phyllograptus series, so high even as the beginning of 

 the range of Diplograptus. This case is the more striking since there 

 are several species of the Scandinavian Shumardia-Ceratopyge fauna 

 (Shumardia pusilla (Sars), Agnostus sidcnbladhi Linnarsson, and Sym- 

 physurus elongatus Moberg and Segerberg) in this limestone high in 

 the Levis. 



Under these conditions, if these species of the Ceratopyge fauna 

 could not arrive in America until late Beekmantown, it is not surprising 

 that many genera which originated in Europe during Beekmantown 

 time, should not have arrived in America till the Chazy. I do not 

 wish to be understood to advocate a general principle that homotaxial 

 formations of separated continents are really one stage apart in age, 

 but each particular case must be decided on its own merits. 



