Finally, in Sharks, the Balistidæ, and others, true scales are absent and replaced by the ossified papillæ of the cutis, which give the surface the appearance of fine-grained chagreen.
Fig. 17.—Dermal papillæ of Monacanthus trossulus.
Fig. 18.—Dermal papillæ of Monacanthus hippocrepis (magn.)
These generally small bodies, as well as the large osseous scutes of the Rays, Sturgeons, etc., have been comprised under the common name Placoid scales; a term which deservedly is being abandoned.
Fig. 19.—Cycloid scale from the lateral line of Odax lineatus (magn.)
Along the side of the body of osseous fishes runs a series of perforated scales, which is called the lateral line (Fig. 21). The perforating duct is simple at its base, and may be also simple at its outer opening (Fig. 19), or (and this is frequently the case) the portion on the free surface of the scale is ramified (Fig. 20). The lateral line runs from the head to the tail, sometimes reaching the caudal fin, sometimes stopping in front of it, sometimes advancing over its rays. It is nearer to the dorsal profile in some fishes than in others. Some species have several lateral lines, the upper one coasting the dorsal, the lower the abdominal outline, one running along the middle as usual.
