232 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 cesophagus. The 
intestine has a spiral valve. 
The cranium, according to Boulenger (‘‘ Poissons du Bassin 
du Congo,” p. 11), 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 
Fic. 166.— Lower ,. 
jaw of Polypte-differ largely from those of bony fishes, ap- 
eae 8 from broaching 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. 
Se le sth eee —_— —— 
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