Chap. XV.] 
SILTJKIAN COLONIES OF BOHEMIA. 
379 
Besides the genera of Trilobites enumerated as occurring in the 1 Primordial ' or 
lowest zone, that band also contains the Hydrocephalus, so named by Barrande 
from its inflated head, and the Arionellus, a large flat form, which Prof. Angelin 
has recognized in the same zone in Sweden. In the succeeding mass of the 
Lower Silurian, or Stage d, is found Beyrichia lata, common also to Sweden 
and America. Acidaspis Buchii has been described also by M. Bertrand Geslin 
from rocks of this age near Nantes ; whilst other Entomostraca, chiefly bivalved 
(of which there are about twenty species), range from the lowest to the highest 
of the Upper Silurian limestones. 
Space does not permit of an examination of the analogies and identities pre- 
vailing among the Brachiopods and Gasteropods of the basin of Prague as com- 
pared with those of Silurian age in other countries. Suffice it to say that, when 
fully elaborated, the whole of the fossils will probably amount, under the dis- 
crimination of M. Barrande, to upwards of 2000 species ! * 
Other results of the researches into the contents of the Bohemian basin show 
just those agreements in the general distribution of life and the peculiarities of 
local arrangement which are found in all synchronous deposits. Thus the Ptery- 
goti in Britain ascend from the Llandovery to the Ludlow rocks ; and in Bohe- 
mia they range upwards from Stage e, or the representative of the Wenlock 
formation. 
Again, the Asteriadse, or Star-fishes, which are rare in what may be called the 
Llandeilo rocks, or Barrande's subdivision d 1 , become conspicuous in the higher 
stage d 4 , or Caradoc. They have the same range in Britain and Canada. 
Notwithstanding their strict conformity to each other, the Lower and Upper 
Silurian rocks of Bohemia are supposed to be more sharply separated from each 
other by differences of their fossils than in the wider area of the British Isles. 
This fact may to a great extent be explained by an appeal to the usual pheno- 
menon of a clear separation of types wherever a limited tract only is surveyed. 
Here, however, we have another cause, by which M. Barrande accounts satis- 
factorily for the specific separation, in this tract, of the animals of his three Silu- 
rian zones. He shows that, after the accumulation of what he calls the ' Pri- 
mordial Zone,' the sea-bottom of this region was powerfully disturbed by the 
eruption of porphyries. Again, after the completion of his Second Zone, or at 
the close of the older Silurian period, other igneous rocks (greenstones &c.) were 
so copiously and repeatedly evolved as to account for the destruction of nearly 
all the animals then living in this area, the creatures which were enabled to 
exist and live on during such turbulent conditions being for the most part 
Graptolites. 
Yet even from this point of view, the author of the ' Bassin Silurien de la 
Boheme ' has proved that not less than sixty species of fossils are common to 
his Stages d and E. Thus, when the observer is working his way up through 
the Lower Silurian schists and quartzites of the Stage d, with their Orthidae 
and Trinuclei, in them he unexpectedly meets with intercalated bands of dark 
shale, mineralogically similar to those of the upper Stage, and in which, out 
of sixty-seven discovered species, sixty are forms that characterize the Upper 
Silurian Stage e. This is the more remarkable, as the fossils otherwise known 
in the Stage d are by no means similar to those of the overlying rocks. 
M. Barrande, broaching an original theory, has suggested that these schists, 
so insulated among the older strata, must be regarded as having been the seats 
of ' Colonies ' of animals which, existing in some other part of the world whilst 
* Geologists will do well to consult the beautiful suite of Silurian fossils from Bohemia in the British 
Museum, collected, classified, and arranged by M. Barrande. 
