Isospondyli 4 1 



orbitosphenoid, have been lately transferred to the Haplomi by 

 Boulenger. This arrangement is probably a step in advance. 



Ganoid traits are present in certain families of Isospondyli. 

 Among these are the gular plate (found in Amia and the Elopidce), 

 doubtless derived from the similar structure in earlier Ganoids ; 

 additional valves in the arterial bulb in the cellular air-bladder 

 of Notopterus and Osteoglossum, the spiral intestinal valve 

 in Ckirocentridce, and the ganoid scales of the extinct Lepto- 

 lepidce. 



The Clupeoidea. The Isospondyli are divisible into numerous 

 families, which may be grouped roughly under three subdivisions, 

 Clupeoidea, the herring-like forms ; the Salmonoidea, the trout-like 

 forms ; and the Iniomi, or lantern-fishes, and their allies. The 

 last-named group should probably be removed from the order of 

 Isospondyli. In the Clupeoidea, the allies of the great family 

 of the herring, the shoulder-girdle is normally developed, retain- 

 ing the mesocoracoid arch on its inner edge, and through the 

 post-temporal is articulated above with the cranium. The fishes 

 in this group lack the adipose fin which is characteristic of most 

 of the higher or salmon-like families. 



The Leptolepidae. Most primitive of the Isospondyli is the 

 extinct family of Leptolepidce, closely allied to the Ganoid families 

 of Pholidophoridce and Oligopleurida. It is composed of graceful, 



FIG. 27. Leptolepis dubius Blainville, Lithographic Stone. (After Woodward.) 



herring-like fishes, with the bones of the head thin but covered 

 with enamel, and the scales thin but firm and enameled on their 

 free portion. There are no fulcra and there is no lateral line. 

 The vertebrae are well developed, but always pierced by the 

 notochord. The genera are Lycoptera, Leptolepis, ALthalion, 

 and Thrissops. In Lycoptera of the Jurassic of China the 



