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 to crushing hard-shelled objects of prey. In some 



