Finally, we find ourselves for the first time called upon to consider the remains of undoubted vertebrate animals, in the form of Fishes.  The oldest of these remains, so far as yet known, are found in the Lower Ludlow rocks, and they consist of the bony head-shields or bucklers of certain singular armoured fishes belonging to the group of the Ganoids, represented at the present day by the Sturgeons, the Gar-pikes of North America, 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:—­