152 



HISTORICAL PALEONTOLOGY. 



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 impor- 

 tance, 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 



Fig. 101. Clymenia SedgwickU . Devonian, Europe. 



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 Elasmobranchii, 



