. The chordates. Chordata. The Inner (Alimentary) Tube and Its Respiratory Derivatives 47 A B C D E. Fig. 45. Types of mammalian teeth. (A) Incisor. (B) Incompletely developed incisor with root widely open. (C) Incisor of a rodent. (D) Human molar. (E) Molar of an ox. (c) Cement; (d) dentine; (e) enamel; (p) pulp-cavity. (Courtesy, Zittel: "Handbuch der Paleontologie," Munich, R. Oldeubourg.) jaw, upper edge of lower)—the acrodont condition; or they may be ankylosed to the inner vertical surface of the jaw—pleurodont. Or, as in crocodilians and mammals, the root may be firmly lodged
. The chordates. Chordata. The Inner (Alimentary) Tube and Its Respiratory Derivatives 47 A B C D E. Fig. 45. Types of mammalian teeth. (A) Incisor. (B) Incompletely developed incisor with root widely open. (C) Incisor of a rodent. (D) Human molar. (E) Molar of an ox. (c) Cement; (d) dentine; (e) enamel; (p) pulp-cavity. (Courtesy, Zittel: "Handbuch der Paleontologie," Munich, R. Oldeubourg.) jaw, upper edge of lower)—the acrodont condition; or they may be ankylosed to the inner vertical surface of the jaw—pleurodont. Or, as in crocodilians and mammals, the root may be firmly lodged in a socket or alveolus in the jawbone, but not fused to it—thecodont (Fig. 46). Teeth and Scales. The teeth of sharklike fishes, in their structure, manner of development, and distribution, exhibit some highly signifi- cant peculiarities. They are small relative to the size of the fish, and very numerous (Fig. 47). The number functional at one time may ex- ceed a hundred. They are arranged in several rows parallel to the jaw. A typical tooth consists of a basal plate enbedded in the oral mem- brane and usually surmounted by a spine which projects through the membrane. The spine consists of dentine capped by a layer of harder substance commonly regarded as enamel, although question has been raised as to whether, in some cases, it is really enamel or merely a variety of dentine. The basal plate has been variously described as being dentine, bone, or cement. It is certainly calcareous. The tooth is produced by interaction of an ectodermal enamel organ and a meso- dermal odontoblast layer, the developing tooth closely resembling a mammalian tooth-germ at an earh stage (Figs. 48, 49). In most sharks the projecting spine of the tooth is pyramidal and more or less elongated and sharp-pointed, with the point directed back- ward. In the flat skates and rays, the spines are usually reduced and the teeth may become broad flat plates close-set to form a pavement, being thus adapted
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