150 HISTORICAL PALAEONTOLOGY. 



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 : (i) The Bony Fishes (Teleostei}, 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 (Gatwidei), comprising 

 the modem 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 Elasmobranchii, 

 comprising the Sharks, Rays, and Chimtera 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 Ganoids. 

 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- 



