ISOSPONDYLI (PRIMITIVE TELEOSTS) 



Contemporary with the lepidosteoids and amioids. of the Triassic and Jurassic were 

 several families that gradually approached and finally attained the basal teleost grade of 

 organization. As noted by Smith Woodward (1895, xix-xxi) and others, their originally 

 rhombic and dense ganoid scales lose the peg-and-socket articulations and become cycloid, 

 overlapping, thin and more or less horny; their tails pass from the hemiheterocercal type, 

 with unexpanded haemal rods, to the primitive homocercal type, immediately prior to the 

 expansion of the hypural bones; the thin, imperfect bony rings of their vertebral column 

 gradually give rise to well ossified centra; meanwhile, the mandible attains simplification 

 by the progressive reduction of the median gular plate and elimination of the sc^called 

 splenial (prearticular); the anterior borders of the fins gradually eliminate the fulcral 

 scales; intermuscular bones appear only in the later members of the series. Thus the old 

 distinctions between ganoids and teleosts are gradually effaced. 



Opercular region of primitive teleosts. — In an excellent comparative study of the bones 

 of the opercular series of fishes, Hubbs (1919, p. 63) writes: 



"The Isospondyli, comprising the oldest and most primitive of the teleosts, retain 

 certain generalized features of the opercular series. Thus, in Elops an intergular plate is 

 developed, and in Jlbula, although the plate itself is lacking, the intergular fold remains. 

 The branchiostegals of the typical Isospondyli (at least the upper ones), persist as thin 

 wide plates. The uppermost and widest ray (which may be termed the branchioperculum, 

 as it seems to be homologous with the plate in Amia to which that name is here applied) 

 is attached closely to the inner margin of the sub- and interoperculum; not having become 

 concealed under these bones, it remains visible from the side. The whole series, in fact, 

 remaining scarcely at all folded together after the fashion of a fan, is visible from below, 

 though the branchial membranes are separate (as they usually are). The plates of the 

 opercular series in the isospondylous fishes differ from those of Amia in the following 

 respects: the reduction of the suboperculum, so that the interoperculum and operculum 

 are in contact anteriorly; the proximal (or anterior) attachment of branchioperculum and 

 branchiopercular fold to the hyoid arch; the more complete imbrication of all the rays; the 

 attachment of branchiostegals to the epihyal as well as to the ceratohyal; the frequent 

 reduction of the rays below the main hyoid suture to rather slender rods, and the occasional 

 attachment of these reduced rays to the edge of the ceratohyal, rather than to its outer 

 face. These last two features are apparently caused by the strong development of the 

 musculus geniohyoideus of the lower jaw, which is attached to the hyoid arch near the suture 

 separating the ceratohyal from the epihyal. The number of the larger and flatter rays 

 attached to the outer surface of the epihyal (the lowermost sometimes on the suture) varies 

 widely in the Isospondyli and related orders [one to ten]; . . . ." 



"... The total number of branchiostegals is three in the Cyprinidae and others, 

 twenty-four to thirty-six in the several species of Elops. Many other figures might be 

 added, but these are enough to illustrate clearly the inconstancy of the number of branchio- 

 stegal rays in the generalized malacopterygian fishes." 



Pholidophorids. — Near the base of the teleost series stands the Triassic and Jurassic 



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