606 The Crossopterygii 



The nostrils, as in true fishes, are superior. The body in these 

 fishes is covered with rhombic enameled scales, as in the gar- 

 pike ; the head is similarly mailed, but, in distinction from the 

 garpike, the anterior rays of the dorsal are developed as iso- 

 lated spines. 



The young have a bushy external gill with a broad scaly 

 base. The air-bladder is double, not cellular, with a large 

 air-duct joining the ventral surface of the oesophagus. The 

 intestine has a spiral valve. 



The cranium, according to Boulenger (" Poissons du Bassin 

 du Congo," p. 1 1), is remarkable for its generalized form, this char- 

 acter forming a trait of union between the Ganoids and the primi- 

 tive Amphibia or Stegocephali. Without considering Polypterus, 

 it is not possible to interpret the homologies of the cranium 

 of the amphibians and the sharks. 



The jaws are similar to those of the vertebrates higher than 

 fishes. Tooth-bearing premaxillaries and dentaries are solidly 

 joined at the front of the cranium, and united by a suture to 

 the toothed maxillaries which form most of the edge of the 

 mouth. Each half of the lower jaw consists of four elements, 

 covering Meckel's cartilage, which is ossified at the symphysis. 

 These are the articular, angular, dentary, and splenial (coro- 

 noid). Most of these bones are armed with teeth. The 

 palato-suspensory consists of hyomandibular, quadrate, ecto- 

 pterygoid, entopterygoid, metapterygoid, and 

 palatine elements, the pterygoid elements bearing 

 teeth. In Erpetoichthys only the opercle is dis- 

 tinct among the gill-covers. In Polypterus there 

 is a subopercle also; the suborbital chain is 

 represented by two small bones. 



The gill-arches are four, but without lower 

 pharyngeals. The teeth are conic and pointed, 



and in structure, according to Agassiz, they 

 FIG. 377. Lower , , . , _ 



jaw of Poiypte- differ largely from those of bony fishes, ap- 



f rom p r oach ing the teeth of reptiles. 



The external gill of the young, first discovered 

 by Steindachner in 1869, consists of a fleshy axis bordered above 

 and below by secondary branches, themselves fringed. In form 

 and structure this resembles the external gills of amphibians. 



