The ganoids pass by degrees into the Iso spondyli, and the soft-rayed fishes pass again by imperceptible gradations into those more specialize 1 forms having spines in the fins structures which are again lost in the most modified members of the same group.
Ganoid traits are present in certain families of Isospondyli. Among these are the gular plate (found in Amia and the pre sumably derived from the similar plate in the earliest ganoids, additional valves in the arterial bulb in Albulida j the cellular air bladder of Notoptertts and Osteoglossum, the spiral intes tinal valve in Chirocentridce, and the ganoid scales of the extinct Leptolepidue. From these characters it is inferred that the soft-rayed fishes are descended from the Lepidostei or Benthosaurida,BatIvipteroida t, Ipnopida , Ronde letticke, Cetomimida, or lantern fishes, Rhinellida (extinct), Dercetida (ex tinct), Chirothissida (extinct), Exocoetoidida (extinct), Maurolicida, or viper fishes, Gonostomicice, Astrotsesthida, Sto ne:Wide, Malacosteida, Plagyodontidce or lancet fishes, Evermannellider, Paralepidida, Stern° p t y chicks and Idacanthida.
The order of Lyopomi (Ain+, loose, n-6ya, opercle), contains a few deep-sea fishes, without mesocoracoid and with the opercular bones distorted. One family, the Haionturithr, with fossil allies.
A series of soft-rayed fishes descended from the Isospondyli are grouped together to form the super-order or series called Ostariophysce. These differ from the Isospondyli in having families of Characida and Erythrinida, the air-bladder being connected, by a series of ossicles called the Weberian apparatus, with the auditory organ.
This series includes the orders of nathi, Heterognathi, Nematognathi, and nonoti, immense groups comprising the vast majority of the fresh-water fishes of the world. The Eventognathi and Heterognathi have the Halecomorphi. ,The more primitive Isospondyli approach more nearly to these ganoid forms than to their later descendants, the catfishes. the eels, or the pikes.
Most ancient of the Isospondyli is the ex tinct family of Leptolepidce of the Triassic. Families mostly still extant, but nearly all of them more or less represented in fossils from the Jurassic on, are the Elopida or tarpons, the Albutida or lady-fishes, the Chaniche or milk fishes, the or moon-eyes, the Spanio dontidce (extinct), the Pachyrhizodontidce (ex the ThryPtodontidce (extinct) the Ptero thrissidce, the Chirocentrida, the Notopterida, the Enchodontida. (extinct) the Ichthyodectida
(extinct), the Osteoglossida', the Phareodon tidee (extinct), the Clupeida' or herrings, the or round herrings, the Doroso or gizzard-shad, the Engrauidce or anchovies, the Alepocephalida, the Pantodon tida, the Salmonidce or salmon and trout, the Thymallida or graylings, the or smelt, the Microstomida., the Sakingida or ice-fish, the Galaxiida. or New Zealand trout, the Hap plo chit o nidce , the G ono rhync hidce , the Notopterida, and a host of other forms, mostly from the deep seas, constituting (sub-order Iniomi) the families of Aulopicla, Ctenothiris sida (extinct), Synodontidce or lizard-fishes, mouth-parts normal, the maxillary not rudi mentary, and the body usually covered with ordi nary scales. In the order Heterognathi (ert differing; yvdOoc, jaw) the lower pharyngeals are not especially modified, and the jaws usually with teeth. This group comprises most of the river-fishes of South America and Africa with multitudes of genera and species. It includes the families of Characida and Erythrinidoe, the former with and the latter without the adipose fin characteristic of catfishes and salmon.
The order Eventognathi ei, well; iv, within; yvaeoc, jaw, is characterized by the absence of teeth in the jaws, and by the high degree of specialization of the lower pharyngeals, which are scythe-shaped, and in typical forms are armed with a relatively small number of highly specialized teeth of peculiar forms and arranged in one, two or three rows. In all the species the gill-openings are restricted to the sides. there is no adipose fin, and the broad flat branchiostegals are but three in number. In all the species the scales, if present, are cycloid, and the ventral fins, of course, abdominal. The modification of the four anterior vertebra and their connection with the air-bladder are essentially as seen in the catfishes. The families of Eventognathi are the Kneriida, Homalopter Cobitsda or loathes; Catostomida or suck ers, and Cyprinide or carp, dace and minnows. The last is the largest family recognized in ichthyology, comprising over 2,000 suedes, al most all confined to the north temperate zone.