. The chordates. Chordata. 424 Comparative Morphology of Chordates Enamel Dentine Epidermis. Fig. 328. Diagrams illustrating the chief types of fish scales. In all the figures anterior is toward the right. (A) Placoid scales of shark: section perpendicular to surface of skin. (B) Surface view of flat, slightly overlapping rhomboid scales. (B') Part of a section perpendicular to the surface of a cosmoid rhomboid scale, much enlarged. The histologic structure of the cosmin suggests that it is produced by fusion of numerous units, each resembling a placoid spine (but without enamel), com- pactly
. The chordates. Chordata. 424 Comparative Morphology of Chordates Enamel Dentine Epidermis. Fig. 328. Diagrams illustrating the chief types of fish scales. In all the figures anterior is toward the right. (A) Placoid scales of shark: section perpendicular to surface of skin. (B) Surface view of flat, slightly overlapping rhomboid scales. (B') Part of a section perpendicular to the surface of a cosmoid rhomboid scale, much enlarged. The histologic structure of the cosmin suggests that it is produced by fusion of numerous units, each resembling a placoid spine (but without enamel), com- pactly crowded together on a thick bony base. (B") Section, perpendicular to the surface of the skin and much enlarged, showing contiguous regions of two ganoid rhomboid scales. (C) Surface view of overlapping teleost scales. (C) A single teleost scale. Kpidermis (e) covers the region of the posterior surface which is not overlapped by other scales. (C") Section, perpendicular to the surface of the body, of overlapping teleost scales. SUBCLASS II. HOLOCEPHALI Endoskeleton cartilaginous except for a persisting notochor«l around which are developed incomplete cartilaginous vertebrae. fin heterocercal. Upper jaw immovably joined to cranium. Most of skin devoid of scales. No spiracles and only four pairs of branchial clefts. The four clefts on a side open under an operculum, a flap of skin extending from in front backward over the gill region. The space beneath the operculum opens by a single lateral aperture at the rear of the gill region. Spiral valve in intestine. No cloaca, the anal and urinogenital passages opening separately on the ventral surface. These fishes are, in general appearance, sharklike (Fig. 329B). They attain a length of 1 to 3 feet and are all marine, having a wide distribu- tion in the oceans of both Eastern and Western Hemispheres. There are only three living genera, of which the best known is Chimaera. Some classifications place the Holocephali a
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