well; and many beds of the Upper Silurian exhibit myriads of such tubes scattered promiscuously over their surfaces. The last and highest group of the _Mollusca_--that of the _Cephalopoda_--is still represented only by _Tetrabranchiate_ forms; but the abundance and variety of these is almost beyond belief. Many hundreds of different species are known, chiefly belonging to the straight _Orthoceratites_, but the slightly-curved _Cyrtoceras_ is only little less common. There are also numerous forms of the genera _Phragmoceras, Ascoceras, Gyroteras, Lituites_, and _Nautilus_. Here, also, are the first-known species of the genus _Goniatites_--a group which attains considerable importance in later deposits, and which is to be regarded as the precursor of the _Ammonites_ of the Secondary period. [Illustration: Fig. 74.--Head-shield of _Pteraspis Banksii_, Ludlow rocks. (After Murchison.)] [Illustration: Fig. 75.--A, Spine of _Onchus tenuistriatus_; B, Shagreen-scales of _Thelodus_. Both from the "bone-bed" of the Upper Ludlow rocks. (After Murchison.)] 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