
Parr IL Sxor. ii §2.] SILURIAN, 687 
Middle Graptolite (Dicranograptus) schists, with Phyllograptus) 
typus, Didymograptus geminus, Diplograptus pristis, D. 
folium, D. teretiusculus, and forms of Asaphus, Ogygia, 
Trinucleus, &e.  . 
Lower Orthoceratite limestone (Ceratopyze-Kalk), with} — Llandeilo. 
Orthoceras duplex, O. annulatum, Lituites cor | 
Orthis calligramma, O. elegantula, Bellerophon bilobatus 
Ptychopyge applanata, Megqalaspis limbatum, ps a | 
bratus, &e. . 
Lower Graptolite (Phyllograptus) schists (with eerie 
graptolites of the genera Didymograptus, Tetragraptus,{ _ 
Dichograptus, Temnograptus, pe ogni, &e., oe on 
the Cambrian Alum-schists} . 
= Arenig in part. 
In the Christiania district, according to Soe the following sub- 
divisions can be established : 
(Compact grey, often bituminous limestone, with abundant Orthoceras coch- 
leatum and Chonetes striatella. 
; |Grey somewhat bituminous limestone, with shales and clays. 
S. Fissile green or grey marly shales containing the last graptolites. This 
| and the two overlying members have a united depth of 835 Norwegian 
feet at Ringerige. 
\Coral-limestone and Pentamerus limestone. 
(Calcareous sandstone, with Rhynchonella diodonta and shales, 150 to 370 feet. 
= |Shales and marls, with nodules and short beds of cement-stone (Trinu- 
= cleus, Chasmops), 700 feet. 
aS |Graptolite shales, Limestone in two or more bands (Orthoceras-, Asaphus-, 
\. Megalaspis-limestone), 250 feet in places. 

Though the general resemblance of the succession of fossils in 
Scandinavia and in Britain is singularly close, there are, as might have 
been anticipated, differences in the range of species, some forms having 
appeared earlier or having survived later in the one region than in the 
other. Thus the Pentamerus oblongus ascends in Scandinavia into rocks 
full of Wenlock corals, but does not occur in the Wenlock group of 
Britain. On the other hand, among Scandinavian strata containing such 
characteristically Lower Silurian genera of trilobites as Asaphus, Trinu- 
cleus, and Ogygia, there occur organisms which in Britain are typically 
Upper Silurian, such as Orthoceras dimidiatum and O. distans, two fossils 
of the Ludlow rocks. In Britain no graptolites have yet been found 
below Arenig rocks, but in Scandinavia they occur in the Dictyonema 
schists, which are probably of Upper Cambrian age. ‘These facts possess 
considerable importance in relation to the value of paleontological 
evidence in correlating the formations of different countries, since they 
indicate that the order of succession found to hold good in one region 
cannot be rigidly applied to others, as is so often attempted by paleon- 
tologists, and that in such cases it is not from individual species so much 
as from the general facies of the fossils that we must draw geological 
parallels. The first appearance and duration of a species have doubtless 
greatly varied in different regions. It is altogether against the analogies 
of nature to hold that a species has everywhere had nearly or precisely 
the same chronological range. 
In the northern regions of Sweden and Norway the Silurian forma- 
tions present a remarkably different development from that just described. 
According to the researches of A. E. Térnebohm they are there represented 
by vast masses of quartzite, mica-slate, gneiss, hornblende-schist, clay- 
slate, and other crystalline rocks. The schists can be seen reposing upon 
recognizable Silurian strata in numerous natural sections, and without 
