Every one sees at a glance that the sharks (Figs. 5, 6, 1) are widely different from all ordinary fishes. Their peculiar outline in general, their unequal-lobed tail, their transverse mouth on the underside of the head, their formidable array of lancet-shaped teeth,
Fig. 6.—Thresher-Shark (Alopias vulpes, Bonaparte).
their fixed gills without gill-covers, their rough skin, their pillow-shaped eggs with long, tendril-like appendages at the corners (Fig. 8), all combine to separate them about as far as possible from typical fishes. And if we compare them with one another, what wonderfully-
Fig. 7.—Hammer-head Shark (Zygœna malleus. Valenciennes); and Sawfish (Pristis antiquorum, Latham).
diversified forms do we see as we pass from the dog-sharks to the mackerel-sharks, and from the latter to the white sharks, and from these to the threshers, and to the hammer-heads, and so on through the whole list!
And who that has studied only the ordinary fishes would at length
