356 A HISTORY OF FISHES 



belonging to the next sub-class, but in the presence of ganoid 

 scales, and in the form of the head-skeleton, pectoral arch, etc., 

 they bear a marked resemblance to the Palaeoniscids, from 

 which they are probably descended. Unfortunately, no fossil 

 remains connecting the Bichirs with their Palaeozoic forebears 

 have yet been discovered. 



The Palaeopterygian Bony Fishes were dominant during 

 Palaeozoic times, held on during the Mesozoic era, during which 

 nearly all the orders became extinct, and are represented to-day 

 by a few scattered remnants, such as the Sturgeons {Acipen- 

 seridae), Spoon Bills (Polyodontidae) , and Bichirs [Polypteridae) . 

 The history of the sub-class Crossopterygii followed exactly the 

 same course, the surviving members being the three genera of 

 Lung-fishes found to-day in Australia, Africa and South 

 America respectively. Three orders of Crossopterygians may 

 be recognised, of which the first two, the Rhipidistia and 

 Actinistia^ contain only extinct forms, while the last, the Dipneusti, 

 includes the existing Lung-fishes as well as a number of fossil 

 genera. Representatives of the first and last of these orders were 

 already in existence in the Middle Devonian epoch, and were 

 contemporaneous with the earliest Palaeoniscids. 



The Rhipidistia occur in Devonian and Carboniferous strata, 

 and include a family, the Osteolepidae, the members of which 

 are of particular interest as representing the probable ancestors 

 of the four-footed, terrestrial vertebrates. The paired fins are 

 rounded and lobe-like, the body is covered with ganoid scales, 

 and the teeth in the jaws are of simple form. In the arrangement 

 of the bones of the skull these fishes present certain resemblances 

 to the earliest known Amphibians, and it is not very difficult 

 to see how their pectoral fins might have been converted into 

 five- toed limbs. The characteristic genus, Osteolepis, occurs in 

 the Old Red Sandstone of Scotland, other members of the 

 family being found in the rocks of Europe and North America. 

 A closely related family, Holoptychiidae, shows the same hetero- 

 cercal tail and two small dorsal fins, but the pectorals have a 

 long, blade-like form, and the ganoid scales have given place 

 to overlapping cycloid scales (Fig. i28a). Holoptychius was 

 widely distributed in the Devonian period, remains occurring 

 in the British Isles, Europe, Greenland, and North America. 

 A portion of a slab from the Old Red Sandstone of Scotland, 

 preserved in the British Museum, contains a large number of 

 remains of these fishes, lying so close together as to suggest 

 that they must have been simultaneously overwhelmed by some 



