=Relationships of Isospondyli.=—For our purposes we may divide the physostomous fishes as understood by Müller into several orders, the most primitive, the most generalized, and economically the most important being the order of _Isospondyli_. This order contains those bony fishes which have the anterior vertebræ unaltered (as distinguished from the _Ostariophysi_), the skull relatively complete, or at least not eel-like, the mesocoracoid typically developed, but atrophied in deep-sea forms and finally lost, the orbitosphenoid present. In all the species the ventral fins are abdominal and normally composed of more than six rays; the air-duct is developed. The scales are chiefly cycloid and the fins are without true spines. In many ways the order is more primitive than _Nematognathi_, _Plectospondyli_, or _Apodes_. It is certain that it began earlier in geological time than any of these. On the other hand, the _Isospondyli_ are closely connected through the _Berycoidei_ with the highly specialized fishes. The continuity of the natural series is therefore interrupted by the interposition of the side branches of Ostariophysans and eels before considering the _Haplomi_ and the other transitional forms. The forms called _Iniomi_, which lack the mesocoracoid and the 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 _Elopidæ_), 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 _Chirocentridæ_, and the ganoid scales of the extinct _Leptolepidæ_. =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, retaining 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. [Illustration: FIG. 27.—_Leptolepis dubius_ Blainville, Lithographic Stone. (After Woodward.) ]