. Animal Life and the World of Nature; A magazine of Natural History. 254 Animal Life. skin of rays and sharks, which give to " shagreen" its peculiar rough surface so admirably adapted to the requirements of the polisher. There is, in fact, an imper- ceptible transition from such skin-granules to true internal teeth, and we thus learn that teeth, like the lining mem- brane of the mouth, were originally simple external structures, which, with their transference to the interior, gradually developed the extra- ordinary variety they now present. I have said that in most fishes the teeth
. Animal Life and the World of Nature; A magazine of Natural History. 254 Animal Life. skin of rays and sharks, which give to " shagreen" its peculiar rough surface so admirably adapted to the requirements of the polisher. There is, in fact, an imper- ceptible transition from such skin-granules to true internal teeth, and we thus learn that teeth, like the lining mem- brane of the mouth, were originally simple external structures, which, with their transference to the interior, gradually developed the extra- ordinary variety they now present. I have said that in most fishes the teeth of the jaws, as they become worn out, are succeeded and replaced by new ones. In many cases, however, especially among the sharks and rays, the mode of replacement is quite different from the one obtaining among crocodiles, where each tooth is replaced by a successor growing beneath its root (see previous article of this series). In the sharks and rays, as well as in many of the enamel-scaled fishes, the teeth are developed on the surface of a more or less complete half-cylinder. In the front of the jaws of sharks (Fig. 2) and throughout those of some rays (Fig. 10) the teeth are arranged in straight rows on this half-cylinder, but on the sides of a shark's jaw the rows are oblique, so that a spiral arrangement occurs. As a consequence of this spiral arrangement, it results that the lowest and youngest of the teeth are actually immediately below the topmost tooth in the same row which they are ultimately destined to replace, the crowns of the two teeth at the top and bottom of the series being, of course, turned in the opposite directions. This remarkable arrangement is well displayed in the largest row of crushing-teeth on the sides of the jaw shown in Fig. 2. As the teeth in use on the margins of the jaws become worn and shed, the whole half-cylinder gradually revolves on the jaw so as to bring a fresh series of teeth into use, while new teeth are developed on the basal line
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