OCR Text |
Show 1 9 0 6 . ] SELACHIAN FISHES. 751 has shown that it is closely related to Squalus. The anatomy of Pristis has been described by Gegenbaur and agrees in all essential features with that of Rhinobatus. In spite of these researches the writers of modern text-books still seem to believe in some special relationship between these two genera, which resemble each other to a certain extent in the appearance of the saw-like rostrum, but differ so widely in other respects. Family 5. Squatinhle. Body depressed. Five gill-clefts on each side, the last in front of the base of the pectoral, which is produced forward external to the gill-clefts, this anterior extension being free from the body. Dorsal fins without spines, situated on the tail; anal fin wanting. Teeth subconical, pointed. Pterygo-quadrate not articulated to the cranium. Vertebral centra calcified. The remarkable genus Squatina, dating from Jurassic and Cretaceous times, is unquestionably related to the Squalidae. It has been regarded as a connecting-link between the Sharks and Rays, but in the strongly depressed form of the body and the backward shifting of the dorsal fins it has gone further than the more primitive of the true Batoids, yet without modification of pterygo-quadrate, hyomandibular, hyoid or pectoral arch from the Squaloid type. The gill-clefts are lateral in position and crowded together. The arrangement of the cartilages of the pectoral fin is far more similar to that found in the Squalidae than to that of the Batoids. The cartilaginous supports of the dorsal fin are arranged as in the Squalidae. The paired praeorbital cartilages typical of the Batoidei are absent. In favour of Batoid relationships may be cited the calcification of the vertebral column, which is rather similar to that of the Rhinobatidae, and the structure of the mixopterygia, which, although extremely similar to those of Squalus, present two features characteristic of the Batoids, viz. the development of a ventral covering piece and the presence of a special glandular body in the glandular sac. To sum up then:-In all essential characters, i. e. position of the gill-clefts, structure of pterygo-quadrate, hyomandibular, hyoid and pectoral arch, arrangement of the cartilaginous supports of the pectoral fin, Squatina is a typical Squaloid. In these and other features it shows specially close relationship to the Squalidae. The depressed body, the extension forward of the pectoral fins, and the backward position of the dorsal fins are Ray-like features of specialisation which do not, however, appear to indicate Batoid relationships. Finally, the structure of the vertebral column and the mixopterygia point to real affinity to the Hypotremata, and we must infer that both they and Squatina have evolved from Sharks similar to the Squalidae, but in which probably the vertebral column and the mixopterygia had already attained a structure somewhat similar to that found in Squatina. P roc. Z ool. S oc.-1906, No. L. 50 |