Sub-order A. Holocephali

This sub-order includes certain curious fishes, of which the only living forms are the Chimaeridae. The notochord is persistent; but the neural arches and transverse processes are cartilaginous. The jaws are bony, and are covered by broad plates representing the teeth. The exoskeleton consists of placoid granules. The first ray of the anterior dorsal fin is in the form of a powerful defensive spine, like the "ichthyodorulites" of many fossil fishes. The ventral fins are abdominal, and the tail is heterocercal. There is only a single external gill-aperture, covered with a gill-cover and branchiostegal membrane; but only a small portion of the borders of the branchial laminae is free. The mouth is placed at the extremity of the head.

The best-known living representative of the sub-order is the Chimaera monstrosa (fig. 269, B), commonly known as the "king of the Herrings."

Sub order A Holocephali 339Sub order A Holocephali 340Fig. 269.   A, Spinax acanthias, one of the Dog fishes. B, Chimaera monstrosa. C, Tail spine of an Eagle ray (Myliobatis).

Fig. 269. - A, Spinax acanthias, one of the Dog-fishes. B, Chimaera monstrosa. C, Tail-spine of an Eagle-ray (Myliobatis).

In Chimaera there is only one apparent gill-slit, but the gills really adhere to the integument by a large portion of their borders, and there are consequently five holes communicating with the gill-slit. A rudimentary operculum is present, covered by the skin. In the closely-allied Callorhynchus from the South Seas, there is a large fleshy appendage at the end of the snout. In the Secondary and Tertiary Rocks are found several fossil forms, constituting the genera Edaphodus, Elasmodus, and Ischiodus.

Sub-order B. Plagiostomi

This sub-order is of considerably greater importance, as it includes the well-known Sharks and Rays. The vertebral centra are usually more or less ossified, and even when quite cartilaginous, the centra are marked out by distinct rings. The skull is in the form of a cartilaginous capsule, without distinct cranial bones. The mouth is transverse, and is placed on the under surface of the head (fig. 268, A). The exoskeleton consists of placoid granules, tubercles, or spines. The branchial sacs open externally by as many distinct apertures as there are sacs, and there is no operculum. A pair of tubes proceed from the pharnyx to open on the upper surface of the head by two apertures, which are termed "spiracles," and which are sometimes regarded as the homologues of the Eustachian tube and external meatus audi-torius (Wyman). By means of these water can be admitted to the pharnyx, and thence to the gills.

By Professor Owen the Plagiostomi are divided into three sections, termed respectively the Cestraphori, the Selachii, and the Batides.