nscEs. 65 



CLASS PISCES. 



This group is so heterogeneous, if we include all the verte- 

 brates below the Amphibians, that it is difficult to characterize 

 it. Broadly speaking, Fishes may be designated as vertebrates, 

 aquatic and water-breathing, with a heart composed of one auricle 

 and ventricle, and with limbs represented by fins. 



The lowest living vertebrate is the aberrant Lancelet {ATnphi- 

 oxus lanceolatus). It is about two inches long, and destitute of 

 skeleton, skull, jaws, fins and heart, and a brain scarcely repre- 

 sented. The notochord is persistent. Some recent writers make 

 this animal the type of a branch, Acrania, of the subkingdom 

 Yertebrata, with all other vertebrates as Craniota. 



Huxley divides fishes into the following orders: 



Dipnoi (Lung-fishes): Ceratodus, Protojyterus, Lepidosiren. 



Elasmobranchii (Placoids) : Sharks and Rays. 



Ganoidei : Sturgeons, Gar-pikes, Mud-fishes, Polypte^nis. 



Teleostei : Bony Fishes. 



Marsipobranchii (Cyclostoniata) : Hag-fishes and Lampreys. 



Pharyngobranchii (Leptocardii) : Lancelet. 



Gunther and Newberry unite the Dipnoi with the Ganoids ; 

 and some recent authors make the Marsipobranchii and Pharyn- 

 gobranchii separate classes. 



The Ganoids and Elasmobranchs -were the earliest fishes, and 

 the latter are still numerous, and the largest and most powerful. 

 These two orders, with Dipnoi, possess the greater number of 

 reptilian features, but the Teleosts are the most highly special- 

 ized, and the characteristic fishes of Tertiary and later time. 



The fossil fishes fall into the above orders. The earliest ver- 

 tebrate and fish remains are fragments of Sharks and Cephalaspids 

 in the uppermost Silurian of England and Europe. The earliest 

 American remains are from the lower Devonian, and in the Cor- 

 niferous, fishes are well represented. Devonian fishes from 

 America are less numerous, but larger than those from Europe. 

 All Devonian fishes, as well as those from the Carboniferous, 

 Triassic and Jurassic, belong to either Dipnoi, Elasmobranchs or 



