merica,
and a few other less familiar forms. The principal Upper Silurian
genus of these is _Pteraspis_, and the annexed illustration (fig.
74) will give some idea of the extraordinary form of the shield
covering the head in these ancient fishes. The remarkable stratum
near the top of the Ludlow formation known as the "bone-bed" has
also yielded the remains of shark-like fishes. Some of these,
for which the name of _Onchus_ has been proposed, are in the form
of compressed, slightly-curved spines (fig. 75, A), which would
appear to be of the nature of the strong defensive spines implanted
in front of certain of the fins in many living fishes. Besides
these, have been found fragments of prickly skin or shagreen
(_Sphagodus_), along with minute cushion-shaped bodies (_Thelodus_,
fig. 75, B), which are doubtless the bony scales of some fish
resembling the modern Dog-fishes. As the above mentioned remains
belong to two distinct, and at the same time highly-organised,
groups of the fishes, it is hardly likely that we are really
presented here with the first examples of this great class. On
the contrary, whether the so-called "Conodonts" should prove
to be the teeth of fishes or not, we are justified in expecting
that unequivocal remains of this group of animals will still be
found in the Lower Silurian. It is interesting, also, to note
that the first appearance of fishes--the lowest class of vertebrate
animals--so far as known to us at present, does not take place
until after all the great sub-kingdoms of invertebrates have
been long in existence; and there is no reason for thinking that
future discoveries will materially affect the _relative_ order
of succession thus indicated.

LITERATURE.

From the vast and daily-increasing mass of Silurian literature, it
is impossible to do more than select a small number of works which
have a classical and historical interest to the English-speaking
geologist, or which embody researches on special groups of Silurian
animals--anything like an enumeration of all the works and papers
on this subject being wholly out of the question. Apart, therefore,
from numerous and in many cases extremely important memoirs,
by various well-known observers, both at home and abroad, the
following are some of the more weighty works to which the student
may refer in investigating the physical characters and succession
of the Silurian strata and their fossil contents:--

(1) 'Siluria.' Sir Roderick M