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In true vegetable-feeding, hoofed mammalia, we find remarkable interblending of dentine, enamel, and cement, in the molars, the proportions of each in different cases varying, as well as the pattern presented by the crown of the teeth. A general idea of such arrangement, and of its consequences, (roughness of surface,) may be obtained by inspection of the old molar teeth of an ox or sheep. In such animals also, owing to the peculiar mechanism of the joint of the lower jaw, there is very free motion in various directions, and thus the whole triturating surface of the teeth, so well adapted to their function, is readily available.

In the elephant, several curious modifications may be observed in the large molar teeth. Each is composed of a number of connected plates, having the usual arrangement of dentine, enamel, and cement substances, the latter being at first the binding material by which the plates are tied together. "The formation of each grinder begins with the summit of the anterior plate, and the rest are completed in succession; the tooth is gradually advanced in position as its growth proceeds, and in the existing Indian elephant, the anterior plates are brought into use before the posterior are formed." In all elephants, the molars succeed each other from behind forwards, moving in a curve, the young growing tooth being nearly at right angles to the one already in use. Professor Owen's statement is so much to the point, that we shall quote it entire, "The jaw is not encumbered with the whole weight of the massive tooth at once, but it is formed by degrees as wanted; the subdivision of the crown into a number of successive plates, and of these into subcylindrical processes, presenting the conditions most favourable to progressive formation. Subdivision

1 Cyclopædia of Anatomy, Art. Teeth.

of the tooth gives another advantage, each part, like a simple tooth, has dentine, enamel, and cement. The different parts of the tooth, as they come forwards, have, of course, from differences of attrition, different kinds of surface, the anterior portion for crushing branches, the middle, with its transverse ridges, for reducing these to smaller fragments, and the posterior tubercles of enamel grind it to a pulp."

Such special modifications of masticating organs, according to the gnawing, flesh-eating, or vegetablegrinding habits in different animals, are sufficiently obvious, but not more so than the peculiarities of organs fitted for mixed food, as in human beings. In his dentition, man presents a character intermediate between the carnivorous and the herbivorous type; "the presence of canines, and the absence of complex structure, arising from interblending of vertical plates of the different dentinal tissues in the molars, would prove that the food could not have been the coarse, uncooked vegetable substances for which complex molars are adapted ; and, on the other hand, the feeble development of the canines, and the absence of molars of the sectorial shape, and opposed like scissors' blades, would equally shew that the species had been unfitted for obtaining habitual sustenance from the raw, quivering fibre of recently killed animals." How evident, therefore, the relation between the kind of food which man naturally makes choice of, and the organs which bruise and prepare it for the act of digestion in the stomach; there is assuredly no mere accidental coincidence in all these arrangements.

The Ophidia, (or serpent order,) as has been already stated, take in their prey entire, and their teeth are generally simply conical, and fitted for retention. The poison

1 Owen's Odontography, p. 471.

fangs of certain species are particularly worthy of notice, and certain non-venomous species are instructive as presenting a transitional form. Some of the teeth in the upper jaw, in certain kinds of serpents, present a longitudinal groove which serves to conduct an acrid, but not deadly saliva, into the wounds which they inflict. The true venomous fang, such as that of the rattlesnake, is just a flat tooth folded on itself, and the edges united; the hollow or canal which traverses it is in communication with the poison-gland at its base, the muscles covering which being in powerful action during attack, compress the gland, and squeeze its deadly contents into the wound inflicted by the fang. When not in use, these formidable weapons are retracted and concealed in a fold of the gum, with their points directed backwards; the relation of tooth, jaw, and other parts, is sufficient to uncover the recumbent fang, and bring it into use when required.

In fishes, special adjustments are as numerous and remarkable as in higher vertebrata. Those that feed on worms, and similar soft food, have teeth which are simply conical, and differing in number and size, according to the minor modifications of habit; the barbel and others present such a form of dental apparatus, well fitted for simple capture and retention of the food. The wolf-fish, again, has a dental furniture suited for bruising the shells of the mollusca on which it feeds; the thin membranous stomach of that species shews that the pavement of bruising teeth, with which its mouth is lined, serves for the effectual comminution of its prey, rendering the presence of a gizzard unnecessary. The fishes which feed on the coral-building animals have parrot-like jaws in front, for breaking off the calcareous polypidoms, and on the pharyngeal bones behind, an apparatus to crush and

prepare for digestion in the stomach. The pharyngeal teeth of the wrasse are each in the form of an arch of great strength, admirably fitted for the process of crushing: "if the engineer would study the model of a dome of unusual strength, and so supported as to relieve from its pressure the floor of a vaulted chamber beneath, let him make a vertical section of one of the crushing pharyngeal teeth of a wrasse."1 In the carnivorous Sphyraena, whose teeth are liable to injury during efforts to secure its living prey, these formidable organs are continually replaced, the alternate teeth being shed cotemporaneously, by which provision is made for having a series of offensive weapons always ready for use.

The position of teeth, also, in this class, is in strict conformity with the habits and general organization. Those of flat fishes (flounders, &c.) are unequally distributed, being most numerous on the side next the under surface of the animal; in other words, that side of the jaws next the ground, (in the usual position of such fishes,) being the one nearest their food, which is under them.

It appears, then, that in these instruments, as in every other part of the animal frame, while a general plan subordinates the whole, there is, at the same time, a vast number and variety of modifications, each in beautiful harmony with the instincts and habits of the animal, with all its organs, and with the place and part assigned to it by its great Author.

1 Owen's Odontography, p. 788.

CHAPTER VI.

MOLLUSCA.

SECT. I.-TYPICAL FORMS OF MOLLUSCA.

WE have, in the preceding pages, been directing attention to animals possessed of an internal skeleton formed of parts constructed according to a common plan. We pass to the examination of others generally characterized by the absence of such a framework, but often presenting hard parts on the outside, constituting exoskeleton.

It is admitted that there are three types of invertebrata: the molluscan, as the oyster, &c. ; the articulate, such as insects and crabs; and the radiate, comprehending the star-fishes, &c.

Although much remains to be done in reducing these departments of the animal kingdom to the same philosophical order which the department of the vertebrata has attained, we shall find no lack of examples for illustrating the argument, some of the more obvious and prominent of which may now be examined. We begin with mollusca.

The investigations of observers on the Continent, and in our own country, have demonstrated that in the earlier periods of life, the mollusca present symmetry of parts in reference to a vertical and longitudinal plane. An examination of the history of development in the

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