150. HISTORICAL PALASONTOLOGY. 
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 import- 
ance, as being more or less extensively represented in past time. 
These groups are: (1) The ony Fishes ( Teleoste’), comprising 
most existing fishes, in which the skeleton is more or less com- 
pletely 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 unsym- 
metrical, being divided into two unequal lobes ; and the scales 
(when present) have the form of plates of bone, usually cov- 
ered 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 Elasmobranchit, 
comprising the Sharks, Rays, and Ciimare of the present day, 
in which the skeleton is cartilaginous; the tail is unsymmetri- 
cally lobed; and the scales have the form of detached bony 
plates of variable size, scattered in the integument. 
It is to the two last of these groups that the Devonian fishes 
belong, and they are more specially referable to the Ganozds. 
The order of the Ganoid fishes at the present day comprises 
but some seven or eight genera, the species of which princi- 
pally or exclusively inhabit fresh waters, and all of which are 
confined to the northern hemisphere. As compared, there- 
fore, with the Bony fishes, which constitute the great majority 
of existing forms, the Ganoids form but an extremely small and 
limited group. It was far otherwise, however, in Devonian 
times. At this period, the bony fishes are not known to have 
come into existence at all, and the Ganoids held almost undis- 
puted possession of the waters. To what extent the Devonian 
Ganoids were confined to fresh waters remains yet to be proved ; 
and that many of them lived in the sea is certain. It was 
formerly supposed that the Old Red Sandstone of Scotland 
and Ireland, with its abundant fish-remains, might perhaps be 
a fresh-water deposit, since the habitat of its fishes is uncer- 
