Pisces. | LOWER PALAZOZOIC VERTEBRATA. 579 
Sect. XI. VERTEBRATA OF MIDDLE PALAZOZOIC (Devonian) ROCKS. 
4th Sub-kingdlom. VERTEBRATA. See page 575. 
Ist Class. PISCES. See page 575. 
4th Order. Ganorpna (Ag. restricted). 
Scales, very rarely thin and without ganoine, usually composed of a thick, bony or corneous layer, covered 
by a dense polished enamel-like external layer of ganoine; usually rhomboidal and finely punctured, in some 
groups rounded, sculptured or spinulose. Hndo-skeleton sometimes ossified, more usually incompletely ossified, 
often not ossified, but the pieces of the head always distinct. Bulbus arteriosus of the aorta with several 
rows of valves, and capable of muscular contraction like a second auricle to the heart. Intestine with a 
spiral valve. 
The recent ganoids have a swim-bladder with an air-duct, and the optic nerves do not cross. Most 
ganoids have “fulcral” scales along the anterior ray of each fin, and no other fishes have them. Most 
ganoids have quadrate bony scales polished with ganoine and articulated to each other, no other fishes having 
the same character. In the numerous valves to the dudbus arteriosus, and in the spiral valve to the intestine, 
these fishes resemble the sharks, but the distinct bones of the head are broadly distinguished from the undivided 
cartilaginous cephalic box of the latter. They are the only operculated fishes in which the bodies of the 
vertebrze are frequently undivided one from the other to form a continuous cord. 
Professor Miiller has separated from this order (as originally defined by Agassiz), Ist, the Lophobranchii 
as a distinct order, having the gills tufted, a small opercular aperture, and no air-duct to the swim-bladder ; 
2nd, the Sclerodermi and Gymnodonti, to form the Order Plectognathi (Cuy.), having the maxillaries and inter- 
maxillaries fixed together, no air-duct, and the exo-skeleton composed of ganoid plates or spines. 
In all the formations below the chalk the Ganoids are the only known bony fishes (except the doubtful 
group of Leptolepis, Thrissops, &c., which Heckel suggests may possibly be Teleostean), being associated in all 
those periods with Placoids. 
The Tribe is divisible into the following Families: 1st, Lepidoidi; 2nd, Acanthodii; 3rd, Sauro-dipterini ; 
4th, Sauroidet ; 5th, Cwlacanthi; 6th, Pycnodonti; 7th, Cephalaspide ; 8th, Placodermata or Pterichthide ; 
9th, Sclerodermata ; 10th, Accipenseride, &c. 
2nd Family. ACANTHODID. 
Body small, short, fusiform, with very minute rhomboidal granulose scales; head large and bony, mouth 
wide, the latter armed with numerous minute teeth, among which a few larger conical ones are irregularly 
interspersed ; eyes large, approximate, on the top of the head; fins of very numerous, fine, articulated rays, 
but generally the first ray in each fin very thick, strong, and bony ; tail heterocercal ; skeleton partially ossified. 
The family is confined to the Devonian and Carboniferous rocks. 
The Genera are Acanthodes, Chirolepis, Chiracanthus, Diplacanthus. 
