LOWER AND UPPER SILURIAN GROUPS CLEARLY DEFINED. 
In the Swedish Upper Silurian group there are, indeed, a few species unknown to 
English geologists. But even these, though wanting in England, are found in 
locks of the same age in other countries. Such, for example, is that peculiar shell 
the Cytherina Baltica, or a variety of it, which has been detected in Normandy and 
Brittany, and also, as we shall hereafter show, in the Timan range of north-eastern 
Kussia. Such also is the Posidonia alata, which is, if we mistake not, a fossil of 
the Clinton division of the Silurian rocks of North America. We cannot make the 
last allusion without observing, that several of the species enumerated, viz. Leptcena 
depressa, L. euglypha, Atrypa tumida, Pentamerus galea tus, Orthis elegantula, Del - 
thyris cyrtama, D. sulcata, Avicula retrojlexa and Hypanthocrinites decorus, as well as 
Calymene Blumenbachi and other Trilobites with some corals, are identical, not only 
with English but also with North American species of the Upper Silurian rocks, 
a sti iking illustration of the wide diffusion of similar conditions in the early stages 
of the foi mation of the surface, of which we shall hereafter adduce many other 
examples. 
To whatever extent, therefore, future researches may prove, that English subdi- 
visions are practicable in it, the Gothlandian group is at any rate a most copious 
and unequivocal display of true Upper Silurian types, which in Sweden are quite 
as distinct from those of the Lower Silurian rocks before described, as in the best- 
studied districts of the British Isles. 
In taking leave of Scandinavia for the present, we must, in the mean time, spe- 
cially advert to the close relations which exist between its Lower and Upper Silu- 
rian groups and those of Great Britain and distant parts of the world. Of 133 
Silurian fossils which we brought back or noted on the spot during our recent 
survey, at least eighty-four are British, and from twenty-five to twenty-seven are 
North American species. In this comparison the identity of the Upper Silurian 
groups of the Baltic and Great Britain is, indeed, most surprising ; for among 
seventy-four Scandinavian species upwards of sixty are common to the strata of 
this age in both countries, and of these fifteen to sixteen species are also found in 
the Upper Silurian rocks of America. 
Such, then, is a brief comparison of the Silurian rocks of Scandinavia with those 
of Great Britain, where their order was first established. In publishing our earliest 
geneial results concerning these deposits we pointedly reminded our brother geo- 
logists, that although in applying the Silurian classification to extensive tracts, the 
minoi and local subdivisions of the typical English region would probably not be 
