The remains of Cephalopoda are far from uncommon in the Devonian deposits, all the known forms being still Tetrabranchiate.  Besides the ancient types Orthoceras and Cyrtoceras, we have now a predominance of the spirally-coiled chambered shells of Goniatites and Clymenia.  In the former of these the shell is shaped like that of the Nautilus; but the partitions between the chambers ("septa”) are more or less lobed, folded, or angulated, and the “siphuncle” runs along the back or convex side of the shell—­these being characters which approximate Goniatites to the true Ammonites of the later rocks.  In Clymenia, on the other hand, whilst the shell (fig. 101) is coiled into a flat spiral, and the partitions or septa are simple or only slightly lobed, there is still this difference, as compared with the Nautilus, that the tube of the siphuncle is placed on the inner or concave side of the shell.  The species of Clymenia are exclusively Devonian in their range; and some of the limestones of this period in Germany are so richly charged with fossils of this genus as to have received the name of “Clymenien-kalk.”

The sub-kingdom of the Vertebrates is still represented by Fishes only; but these are so abundant, and belong to such varied types, that the Devonian period has been appropriately called the “Age of Fishes.”  Amongst the existing fishes there are three great groups which are of special geological importance, as being more or less extensively represented in past time.  These groups are:  (1) The Bony Fishes (Teleostei), comprising most existing fishes, in which the skeleton is more or less completely converted into bone; the tail is symmetrically lobed or divided into equal moieties; and the scales are usually thin, horny, flexible plates, which overlap one another to a greater or less extent. (2) The Ganoid Fishes (Ganoidei), comprising the modern Gar-pikes, Sturgeons, &c., in which the skeleton usually more or less completely retains its primitive soft and cartilaginous condition; the tail is generally markedly unsymmetrical, being divided into two unequal lobes; and the scales (when present) have the form of plates of bone, usually covered by a layer of shining enamel.  These scales may overlap; or they may be rhomboidal plates, placed edge to edge in oblique rows; or they have the form of large-sized bony plates, which are commonly united in the region of the head to form a regular buckler. (3) The Placoid Fishes, or Elasmobranchii, comprising the Sharks, Rays, and Chimoeroe of the present day, in which the skeleton is cartilaginous; the tail is unsymmetrically lobed; and the scales have the form of detached bony plates of variable size, scattered in the integument.