Fig. 248.—Dipterus valenciennesi Agassiz, a Dipnoan. (After Dean, from Woodward.)
Mesozoic Fishes.—In the Triassic period which follows the Permian, the earliest types of Ganoids give place to forms approaching the garpike and sturgeon. The Crossopterygians rapidly decline. The Dipnoans are less varied and fewer in number; the primitive sharks, with the exception of certain Cestracionts, all disappear, only the family of Orodontidæ remaining. Here are found the first true bony fishes, doubtless derived from Ganoid stock, the allies and predecessors of the great group of herrings. Herring-like forms become more numerous in the Jurassic, and with them appear other forms which give the fish-fauna of this period something of a modern appearance. In the Jurassic the sharks become divided into several groups, Notidani, Scyllioid sharks, Lamnoid sharks, angel-fishes, skates, and finally Carcharioid sharks being now well differentiated. Chimæras are still numerous. The Acanthodei have passed away, as well as the mailed Ostrachopores and Arthrodires. The Dipnoans and Crossopterygians are few. The early Ganoids have given place to more modern types, still in great abundance and variety. This condition continues in the Cretaceous period. Here the rays and modern sharks increase in number, the Ganoids hold their own, and the other groups of soft-rayed fishes, as the smelts, the lantern-fishes, the pikes, the flying-fishes, the berycoids and the mackerels join the group of herring-like forms which represent the modern bony fishes. In the Cretaceous appear the first spiny-rayed fishes, derived probably from herring-like forms. These are allies or ancestors of the living genus Beryx.
Fig. 249.—Hoplopteryx lewesiensis (Mantell), restored. English Cretaceous. Family Berycidæ. (After Woodward.)
Dr. Woodward observes:
"As soon as fishes with a completely osseous endoskeleton began to predominate at the dawn of the Cretaceous period, specializations of an entirely new kind were rapidly acquired. Until this time the skull of the Actinopterygii had always been remarkably uniform in type. The otic region of the cranium often remained incompletely ossified and was never prominent or projecting beyond the roof bones; the supraoccipital bone was always small and covered with the superficial plates; the maxilla invariably formed the greater part of the upper jaw; the cheek-plates were large and usually thick; while none of the head or opercular bones were provided with spines or ridges. The pelvic fins always retained their primitive remote situation, and the fin-rays never became spines. During the Cretaceous period the majority of the bony fishes began to exhibit modifications in all these characters, and the changes occurred so rapidly that by the dawn of the Eocene period the diversity observable in the dominant fish-fauna was much greater than it had ever been before. At this remote period, indeed, nearly all the great groups of bony fishes, as represented in the existing world, were already differentiated, and their subsequent modifications have been quite of a minor character."
Fig. 250.—A living Berycoid fish, Paratrachichthys prosthemius Jordan & Fowler. Misaki, Japan. Family Berycidæ.