PENGELLY — ON THE DEVONIAN AGE OF THE WORLD. 
337 
Amorpliozoa and others ; others rich then are poor now, as Brachio- 
poda and Cephalopoda : whilst Gasteropoda, Lamellibranchiata, &c., 
abounding- in species in existing seas, were formerly by no means 
thus characterized.* In order to show this numerical relation of 
the Devonian and existing species, the fifth column of figures, headed 
Dev. Spe., Liv. Spe., in the table, has been calculated thus : — the 
number of species in each class in the existing fauna, has, for the 
present purpose, been regarded as normal and put = 1,000, and 
the number in each of the Devonian classes equated to this ; so that 
when compared with the specific development of the classes of the 
present day those of the Devonian age of the world stand, in ascending 
order, thus : — Cirripedia, Annelida, Pisces, Gasteropoda, Amor- 
phozoa, Lamellibranchiata, Crustacea, Bryozoa, Echinodermata, 
Zoophyta, Pteropoda, Cephalopoda, and Brachiopoda. It will be 
seen also that the number of species in the two last exceeds, and in 
a high ratio, those of the same classes in existing seas ; whilst those 
#f Gasteropoda and Lamellibranchiata are more than correspondingly 
abnormally small. Here we have an example of a high class — 
Cephalapoda — preceding a lower one — Gasteropoda. 
Though when the general fossil census was last taken, the Devonian 
rocks throughout the world yielded so many as one thousand four 
hundred and sixty-eight species ;t yet if this number is considered in 
relation to the great thickness of the deposits of the period, the 
Devonian strata are poorer in species than either the Carboniferous 
or Upper and Middle Silurian ; for example, for every one thousand 
feet in thickness the British Middle Silurian beds contain seventy-nine 
species ; Upper Silurian ninety-six ; Devonian forty-four ; and Car- 
boniferous one hundred and twenty. As a rule, deposits charged 
with peroxide of iron are poor in fossils ; the red limestone of 
Petit Tor near Torquay, however, is an exception to this, as it is 
frequently crowded with Orthoceratites and other Cephalopods. 
It is usual to divide the Devonian system into Lower, Middle, and 
Upper groups, and this triple division has been applied to Devon 
and Cornwall, especially by Professor Sedgwick, who recognizes the 
first, or lowest, in the slates and limestones extending from Plymouth 
to Torquay, in the limestones of Ilfracombe and Linton, the red 
sandstones of the north coast, and in the slates of Looe, Polperro, 
and Powey, in Cornwall. This he designates the " Plymouth group." 
The middle division consists of the slates extending from Dart- 
mouth to the metamorphic schists of the Start and Bolt and the 
slates and purple and greenish sand-rock, stretching in North Devon 
from Morte Bay, east and west across the country : this is termed 
the "Dartmouth group," and is probably without fossils. 
The upper includes the rocks ranging from Baggy point by 
Barnstaple, and the limestone beds and fossiliferous slates of South 
Petherwin in Cornwall : this is known as the Barnstaple or 
* Ibid., 1st and 2nd column of figures, 
t See " Total" Table 2nd column of figures. 
VOL. IV. 2 M 
