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. 

 C.audal 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 as a subdivision under 

 Elasmobranchii. 



SUBCLASS III. CROSSOI'TKRYGII 



Endoskeleton more or less nearly completely ossified. Skull 

 usually strongly ossified, including both dermal and cartilage bones 



