RAYMOND: CORRELATION OF THE ORDOVICIAN STRATA. 269 
Scandinavia, but 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. Having 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 Dictyonema flabelliforme, but 
below that of Tetragraptus and Phyllograptus. The Shumardia 
limestone of America, however, (at Point Lévis) 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 sidenbladhi Linnarsson, and Sym- 
physurus elongatus Moberg and Segerberg) in this limestone high in 
the Lévis. 
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. 
