584 PISCES. 



The Order Placodermata had the head and thoracic region inclosed in sculp- 

 tured, bony plates. In some the tail was naked, in others it was covered with 

 ganoid scales ; in some the fins were inclosed in plates, but the vertebra were not 

 ossified. This order includes the oldest fish remains known to the geologist. 



SUBCLASS SELACHIA. 



The word Selachia is derived from selaclios, the Greek word for shark. This 

 subclass is also called Elasmobranchia and Chondropterygia, and it includes the 

 living sharks, rays, and skates. The skeleton is cartilaginous, and the plates of 

 the skull are united without sutures. There are pectoral and ventral fins, and 

 the caudal fin is usually heterocercal. The surface of the body is naked or cov- 

 ered with calcified papillae, comparable with teeth, and even spinous. The pla- 

 coid scales sometimes form a sculptured armor. The dermal spines found fossil 

 are collectively known as Ichthyodorulites. The teeth are never inserted into the 

 jaws, but are sustained in their position by the strong skin of the gums. They 

 sometimes have obtuse crowns, and form a pavement for both jaws ; in other cases 

 the teeth are conical, sharp, arranged in rows, with the apices pointed backward. 

 The subclass is divided into the Holocephala and Plagiostomata. The Holocephala 

 is represented in the existing seas by the Family Chimaeridae, and, it is said, com- 

 bines some of the characters of the Selachia, Ganoidea, and Batrachia. The Pla- 

 giostomata is divided into two orders, the Squalina and Raiina, The vertebrae are 

 well developed, and the skin is covered with plates, shields, or spines. The Order 

 Squalina includes the ocean sharks and dog-fishes. The Order Raiina includes the 

 skates and rays of the present seas; one of them is called the saw-fish, and another 

 produces dangerous electrical discharges. 



SUBCLASS DIPNOA. 



This subclass is said to furnish a connecting link from the Ganoidea to the Ba- 

 trachia. In external appearance the fish are ganoid-like. The body is long, eel- 

 like, covered with scales, and terminates in a compressed caudal fin with weak fin- 

 rays. The head is broad and flat. There are two orders, Monopneumonia and 

 Dipneumonia. The Monopneumonia includes the Ceratodidae, some of which are 

 living in Australia, and they are common in the Mesozoic rocks, but the existence 

 of them in the Palaeozoic rocks is very doubtful. 



The Order Dipneumonia contains the living Family Sirenidae, which contains 

 two genera, the Lepidosiren, from the rivers of Brazil, and the Protopterus, from 

 tropical Africa. There is little reason to believe this order is represented in Palaeo- 

 zoic rocks, though Ctenodus and Dipterus have been referred to it. 



SUBCLASS GANOIDEA. 



Order Acanthodea. 

 Family Acanthodid^e. — Acanthodes. 



Order Chondrostea. 

 Family Chondrosteid^:. — Asterosteus, Macropetalichthys. 

 Family Pal^eoniscid^e. — Chirolepis, Mecolepis, Palaeoniscus, Rhadinichthys. 



