PfSCES. 583 



known that the whole structure of animals must be taken into consideration in clas- 

 sification, his system, like that of every other based on a single character, has given 

 way to more perfect knowledge of animal life and physical structure. And it is 

 quite probable, if the characters of Devonian fishes could be completely ascertained, 

 as we know the living forms, they would all be found to belong to extinct sub- 

 classes. They are judged, however, only from meager fragments of the ossified 

 parts, and arranged by homologies with the existing species, and classed in orders 

 where the affinities seem most strongly to arrange them. 



SUBCLASS GANOIDEA. 



The Ganoidea (from ganos, brightness, in allusion to the enameled armor with 

 which some of them are covered) commences in the Devonian strata, where the 

 fossil remains soon become abundant, and continue to occur from that time forward 

 to the present, though very few families now exist. Agassiz included as Ganoids 

 all fish covered, in whole or in part, with bony plates ; but some of the living genera 

 were found to belong to the Teleostia, and later classification has been held to 

 include all fossil species falling within the original definition of Agassiz and part 

 of the living forms. The dermal skeleton consists of smooth, bony plates, covered 

 with enamel. In some cases they are rhomboidal, arranged edge to edge in oblique 

 transverse rows; in other cases the scales are rounded ; and in a few species the skin 

 is naked. There is much diversity in the skeletons, and all shades of ossification in 

 the vertebral column and skull from cartilaginous to perfect bone. The subclass 

 has been divided into seven orders, viz. : Chondrostea, Halecomorpha, Ginglymoda, 

 Pycnodonta, Crossopterygia, Acanthodea, and Placodermata. 



The Order Chondrostea includes the sturgeons of fresh and salt water, and 

 the paddle-fish or spoon-bill cat of the Mississippi River and its tributaries. 



The Order Halecomorpha (shad-like) is generally united with the Ginglymoda, 

 under the name of the Holostea ; but is distinguished by having large, round scales, 

 no shingle-like fulcra on the fins, and in having the vertebrae concave at both ends, 

 as in the Teleostia. The only living genus is the Amia, called bow-fin, mud-fish, 

 dog-fish, etc. It is common to the lakes and sluggish rivers. The order is not 

 certainly known in Palaeozoic rocks. 



The Order Pycnodonta has a short, vertically-flattened body, covered with 

 rhomboid scales and peculiar dermal ribs. Tail either heterocercal or homocercal. 



The Order Ginglymoda has a bony skeleton, rhomboid scales, and shingle-like 

 fulcra on the fins. The vertebrae are convex in front and concave behind, forming 

 ball and socket joints ; tail heterocercal, and ventral fins between the pectorals and 

 anals. This order is represented by the gar-pikes, which are common in Amer- 

 ican rivers. 



The Order Crossopterygia is represented by two genera in the African waters, 

 and fossils are referred to it back in geological time as far as the Devonian. The 

 scales may be cycloid or rhomboid ; the throat is protected by two or more plates ; 

 the caudal fin is diphycercal ; dorsal fin is divided in two or more divisions^ pec- 

 torals and ventrals have a scaly axis ; no fulcra. 



The Order Acanthodea had cartilaginous skulls, heterocercal tails, rhomboidal 

 scales, and were armed with a spine before each fin, and are said to occupy a;place 

 between the Gauoidea and Selachia. They are all Palaeozoic. 



