1901-1902. | The Teeth of Fishes. 273 
The human tooth is formed of three constituents—enamel, 
dentine, and cementum. The enamel covers the crown, the 
dentine forms the body, and the cementum envelops the 
roots. Teeth are developed by a dipping down of the oral 
epithelium from the surface of the jaw. There are four 
_ methods of attachment: (1) Fibrous membrane, as in sharks 
and rays. The teeth are here attached to fibrous membrane, 
and have no connection with the cartilaginous jaw. (2) 
Hinge, as in pike and hake. (3) Anchylosis, where there is 
membrane between tooth and bone, as is very well seen in the 
section of tooth and portion of the jaw of a python now 
exhibited, showing the marked difference in character between 
the bone of attachment and the rest of the bone. (4) Im- 
plantation in bony sockets, as in most mammals. There is 
here a special development of bone around the tooth, called 
alveolar border. This grows up with the tooth and disappears 
when it goes away. There is a membrane between tooth and 
bone. 
Fishes are divided into four classes: the principal, from the 
point of view of teeth, are—(1) Teleostei; (2) Elasmobranchi. 
The Teleostei are best known to us. They possess a bony 
skeleton, and the mouth is crowded with teeth, pharynx as 
well as jaws. In the pike the teeth are inclined backwards, 
and are larger in some places than others. In the Elasmo- 
branchi, or sharks and rays, the skeleton is formed of carti- 
lage. The teeth are numerous, and are attached to fibrous 
membrane. The horny teeth of the saw-fish are dermal 
spines, are socketed—which is uncommon among fish—and 
grow from persistent pulps. 
_ In the Batrachia the teeth are not so numerous as in fish. 
Toads are edentulous. The frog has a single row of teeth in 
the upper jaw, and the lower jaw, which has no teeth, passes 
inside the upper. The Chelonia—tortoises and turtles—have 
no teeth, the margin of the jaws being sheathed in horny 
cases. 
_ Saurian Reptiles—lizards. In some cases these have small, 
rounded, conical teeth, and in others long and pointed. The 
teeth are anchylosed to the bone by bone of attachment. The 
Succession of teeth is constant. They come from the inside of 
the bases of the old teeth, and pass to the front. 
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