598 



NA TURE 



[October i i, 1906 



of fins ; but as these were unsuited for further elabor- 

 ation, the primitive fishes of this grade did not advance 

 further. They became long-bodied or almost eel-shaped 

 before their final extinction. 



Fishes only began to make real progress when their 

 fin-flaps were stiffened by internal rods of cartilage in 

 addition to the hard skin-structures. Such fins were 

 essentially paddles, and could be used for crawl- 

 ing in the mud as well as for ordinary swimming 

 in water. It is therefore interesting to observe 

 that during the Middle and Upper Devonian 

 periods, when four-legged lung-breathers must 

 have been just beginning to appear on the land, 

 nearly all the highest fishes had their fins in the 

 shape of paddles (Fig. 3, a). It seems as if at 

 (hat time there was a general tendency for the 

 fashionable and most advanced fishes to become 

 crawlers rather than swimmers ; and there can- 

 not be much doubt that the known Crosso- 

 pterygii, or " fringe-finned ganoids," as these 

 fishes are commonly termed, are the unsuccessful 

 survivors of the race which originally produced 

 the earliest crawling lung-breathers or Laby- 

 rinthodonts. The Dipnoi, or paddle-finned fishes, 

 which breathe both by gills and by a modified 

 air-bladder (almost a lung), were also especially 

 abundant at the same period. In fact, in having 

 the fundamental part of the upper jaw fused with 

 the skull instead of loosely suspended from it, the 

 Dipnoi agree more closely with the land animals 

 than do the Crossopterygii ; but before this 

 feature had been acquired, the roof-bones of the 

 skull had subdivided into smaller plates, such as 

 could not have changed into the skull-bones of 

 an ordinary Labyrinthodont, while the teeth had 

 curiously clustered into plates, so that they could 

 never have produced the Labyrinthodont dentition. 

 The few survivors both of Crossopterygians and 

 Dipnoans at the present day. exhibit the usual 

 long-bodied or eel-shaped contour of decrepit 

 derelicts. 



The next grade of fishes, the Chondrostei 

 (Fig. 3, b). which specially characterised the 

 Carboniferous and Permian periods, had fins in 

 which the internal cartilages formed only an 

 effective basal support, while the greater part of 

 iheir expanse was stiffened by flexible skin-fibres, 

 which had become " fin-rays." Some of these 

 fishes degenerated into eel-shaped creatures in the 

 Triassic, Rh.Tetic, and Liassic periods, while 

 others grew to unwieldy proportions and eventu- 

 ally passed into the modern sturgeons. 



Thus far there had been scarcely any ossifi- 

 cation of the internal skeleton of the head and 

 trunk in fishes ; but by the dawn of the Triassic 

 period a large number of the Chondrostei had 

 passed into the Protospondyli, and then the form- 

 ation of a hard brain-case and vertebral column 

 began. This only happened after the median fins 

 had become absolutely complete, namely, after the 

 upper lobe of the tai]^ had shortened so that the 

 i:iil-fin formed a. flexible fan-shaped expansion at 

 the blunt end of the body, while each separate 

 ray in the other median fins was provided with 

 its own definite support. The Protospondyli 

 (Fig. 3, c) characterised the Triassic, Rhaetic, 

 and Jurassic periods, and exhibited endless 

 variety ; but their sole survivors at the present 

 day are the long-bodied Lepidoeteus and Amia 

 of .\merican fresh waters. 



Associated with almost the earliest Proto- 

 spondyli, there were a few precocious fishes 

 which evidentlv completed their vertebral 

 column at once. This race, including such genera as 

 Pholidophorus and Leptolepis, seems to have temporarily 

 exhausted itself in the effort, for it always occupied 

 a secondary place in the fish-faunas until the beginning 

 nf the Cretaceous period, when it rapidly multiplied, 

 became fashionable, and replaced the Protospondyli. 

 Thus arose the modern fishes of the same grade as the 



NO. 1928, VOL. 74] 



herring and salmon, characterised, not only by a complete 

 vertebral column, but also by a simplified lower jaw, which 

 consists only of two pieces on each side (without the 

 splenial bone which forms so conspicuous a feature of the 

 earlier fishes). The Isospondyli, as they are termed, being 

 thus provided with a completely bony internal skeleton as 

 well as completed fins, admitted of many more variations 



3. — Diagram illustrating grades in the evolution of bony fishes. — A, Paddle- 

 finned fish (Rhizodont Crossopterygian) characteristic of the Middle and Upper 

 Old Red Sandstone periods, internal skeleton only partially shown in drawing : 

 tendency \.ov42.rii=, shortening lobes of fins and simplifying their internal supports- 

 B, Ray-finned fish (Palseoniscid) characteristic of the Carboniferous and Permian 

 periods, showing the extended pelvic fin with numerous supports, the dorsal and 

 anal fins with supports fewer than rays, and the caudal fin heterocercal ; toiHcncy 

 towards shortening upper lobe of tail, and towards equality in number between 

 rays and their supports in the other median fins, c, Ray-finned fish (Dapedius) 

 characteristic of the Triassic and Jurassic periods, showing short-based pelvic fin 

 with one large support, the dorsal and anal fins having a separate support for each 

 ray, and the caudal fin almost homocercal ; tendency towards acquisition of bony 

 vertebree and ossification of the cartilaginous skull. D, Modern ray-finned bony 

 fish (Hoplopteryx) characteristic of the Upper Cretaceous and Tertiary periods, 

 showing premaxilla below maxilla, completed internal skeleton, pelvic fins far 

 forwards, and some spinous fin-rays ; tendency towards extreme development of 

 ear-capsules, supraoccipital bone, and premaxilla, besides a fixed number of 

 spmoiis fin-rays and the forward position of the pelvic fins. 



than any of their forerunners. The typical fish-head now 

 began, for the first time in its history, to exhibit essential 

 changes. The supraoccipital bone often grew upwards to 

 project on the roof, and thrust outwards the now well- 

 ossified and enlarged ear-capsules (Chirocentridce) ; while 

 the premaxilla sometimes extended backwards to slip 

 beneath the maxilla and exclude the latter from the margin 



