326 ORDERS OF FISHES. 



obtusely conical crown, which is broader than its peduncle of 

 support" (Young). The Pycnodonts are mainly Oolitic, but 

 a few Cretaceous species are known, and one form has been 

 described from the Eocene Tertiary. The Triassic Placodus, 

 formerly referred to this family, is now known to be truly 

 Reptilian. 



SUB-ORDER D. CROSSOPTERYGID^E. "Dorsal fins two, or, 

 if single, multifid or very long ; the pectoral, and usually the 

 ventral, fins lobate ; no branchiostegal rays, but two principal, 

 with sometimes lateral and median, jugular plates, situated 

 between the rami of the mandible ; caudal fin diphycercal, or 

 heterocercal ; scales cycloid or rhomboid, smooth or sculp- 

 tured." (Huxley.) 



All the Ganoids of this sub-order are pre-eminently dis- 

 tinguished by the structure of the paired fins, the pectorals 

 always, and the ventrals usually, consisting of a central lobe 

 or stem, which is covered by scales, and to the sides of which 

 the fin -rays are attached. The nearest approach to this 

 structure amongst living fishes is found in the paired fins of 

 the Barramunda (Ceratodus Fosteri) of the rivers of Queens- 

 land. In this singular fish, which is at present referred to the 

 order of the Dipnoi, the pectoral and ventral fins are supported 

 by a median, many-jointed, cartilaginous rod, to which nume- 

 rous lateral branches are attached. The scales in this sub- 

 order are sometimes rhomboidal, not overlapping one another; 

 at other times they are cycloidal in shape, and are arranged 

 in an imbricate manner. 



Professor Huxley divides the Crossopterygida into the fol- 

 lowing families. (See ' Memoirs of the Geological Survey of 

 Great Britain. Decade X. 5 ) :- 



Fam. I. POLYPTERII. 



Dorsal fin very long, multifid ; scales rhomboidal. 

 Polypterus (fig. 282). 



Fam. 2. SAURODIPTERINI. 



Dorsal fins two ; scales rhomboidal, smooth ; fins sub -acutely 

 lobate. 



Diplopterus, Osteolepis (fig. 282), Megalichthys. 



Fam. 3. GLYPTODIPTERINI. 



Dorsal fins two ; scales rhomboidal or cycloidal, sculptured ; 



pectoral fins acutely lobate j dentition dendrodont. 

 Sub-fam. A. with rhomboidal scales. 



Glyptol&mus (fig. 287), Glyptopomus, Gyroptychius. 

 Sub-fam. B. with cycloidal scales. 



Holoptychius (fig. 288), Glyptolepis, Platygnathus \Rhizodus, 

 Dendrodus, Cricodus, Lamnodus}. 



