Table of Contents
The most obvious external character by which all the sharks, skates, and rays are distinguishable from all of the bony fishes is that they have five or more gill openings on either side of the head, instead of only one. They recall the lampreys in this respect, but it is a commonplace that their jaws and teeth are extremely well-developed. Their skins are tough, and are studded in most of them with denticles (placoid scales), which are not homologous with the scales of bony fishes, for both dermis and epidermis take part in their formation, instead of the dermis alone. The teeth of the sharks and rays represent placoid scales that are modified and are embedded in the gums alone, not in the jaws. The fins are supported at their bases by segmented cartilaginous rods, supplemented in all of the sharks, and in some of the rays by numerous slender horny fibers further out, instead of by rays or spines of the sorts that are seen in the bony fishes. All of their fins are covered with the same leathery skin that clothes the body. Among sharks the tail is uneven ("heterocercal"), with the vertebral column extending out into its upper lobe, but it is whip-like in most of the skates and rays, with no definite caudal fin. The torpedo is an exception to this rule.
The modern representatives of the subclass may be grouped in two orders, the one (Selachii) to include all living sharks, the other (Batoidei) to include the sawfishes, the skates and the rays. They are separated one from the other by the following external differences, and there are skeletal differences between them as well:
|1.||The gill openings are at least partly on the sides; the edges of the pectoral fins are not attached to the sides of the head in front of the gill openings; the upper edges of the orbits are free from the eyeballs, so that they form free eyelids||—||Sharks, (p. 15)|
|The gill openings are entirely on the lower surface; the edges of the pectoral fins are attached to the side of the head in front of the gill openings; the upper edges of the orbits are attached to the eyeballs so that they do not form free eyelids||—||Sawfishes, skates and rays, (p. 57)|
 For further discussion, see Bigelow and Schroeder, Fishes Western North Atlantic, Pt. 1, ch. 3, 1948, p. 64.