VERTEBRATA: PISCES 951 



Pisces. 



The Cyclostomata (Agnatha) represented to-day by the lam- 

 preys, appear to have had some representatives in the remarkable 

 Palaeospondylus of the Old Red Sandstone. The Conodonts have 

 been regarded as teeth of myxinoids. The elasmobranchs, or 

 sharks, were well represented in the Devonic and later beds, the 

 first three orders being wholly confined to the Palaeozoic. The 

 endoskeleton is more or less cartilaginous, the exoskeleton and 

 teeth structurally identical (placoid scales). Generally only 

 teeth, calcified vertebrae and dermal spines are preserved. The 

 true sharks and rays (Plagiostoiiii) are mostly Mesozoic and later, 

 but examples from the Carbonic and even the Mississipic are 

 known. The chimseras, however (HolocepJiali), had representa- 

 tives from the Devonic on. 



The ganoids are remarkable in that their trunk and tail are 

 usually covered with scales, consisting of a thick bony inner layer, 

 and an outer layer of enamel, the scales being in some groups articu- 

 lated by a peg-and-socket arrangement, and in others overlapping. 

 The skull is covered with dermal bones, or completely ossified. 

 The vertebral column is cartilaginous or shows various degrees of 

 ossification. Most of the Palaeozoic ganoids belong to the order 

 Crossopterygii or fringe-finned ganoids. Such are the Devonic 

 genera Holoptychius and Osteolepis — and numerous Mississippic 

 to Permic genera. Other crossopterygians occur in the Mesozoic, 

 and two genera (Polypterus and Calamoichthys) are still living in 

 the rivers of tropical Africa. The cartilaginous ganoids {Chon- 

 drostci) range from the Mesozoic to the present time, a number of 

 genera being still extant, such as the sturgeons and paddle-fish. 



A considerable number of Palfeozoic forms also belong to the 

 Heterocerci, an order ranging from the Devonic (Cheirolepis) to 

 the Upper Jurassic (Coccolepis of the Lithographic beds). Many 

 Carbonic and Permic species (including Palasoniscus, Platysomus, 

 etc., the common forms of the Kupferschiefer of Thuringia, etc.), 

 belong to this order, as well as the Triassic Catopterus of North 

 America. The Lepidostei include the "bony pikes" (Lepidosteus) 

 of the North American rivers, and many Cenozoic and Mesozoic 

 genera, but only one genus (Acentrophorus) has Permic representa- 

 tives. Here belong the widely distributed Triassic Semionotus and 

 the many common Jurassic genera (Dapedius, Lepidotus, Eugna- 

 thus, Caturus, etc.), the order being at its height at that time. 

 The Ainioidei also have a surviving genus (Amia) in the rivers of 



