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352 DENTAL ANATOMY.
lay down some broad principles in regard to dental evolntion, at least
among certain groups of the Mammalia, where they have been subjected
to the greatest amount of modification.
Although there are many questions concerning the origin and details
of tooth-evolution of many aberrant forms which remain to be solved,
yet the discoveries which have been made in palseontology within the
last twenty-five years leave scarcely a living group of animals, the
development of whose teeth has progressed beyond the primitive stages,
from which we have not gained some imj)ortant information relative to
the phases through which they have passed to reach their present con-
dition. The possibility of reducing our knowledge of the dental struc-
tures of the Mammalia to a broad and comprehensive basis was long
since recognized by Prof. Cope, to whom probably more than any one
else we are indebted for a genuine philosophic insight into the forms
and structure of these teeth. Scarcely less important are the contribu-
tions of John A. Ryder and Dr. Harrison Allen, whose learned researches
into the probable causes of tooth-modification have marked notable
stages in the progress of the sulyect and have opened new and inter-
esting fields for investigation. Nor should we omit a mention of the
researches of Flower, nor those of Tomes, Waldeyer, Frey, Hertwig,
Magitot, and Legros, into the histology and development in later
times.
Commonly, teeth are defined as hard bodies attached to the parictes
of the mouth or oral extremity of the alimentary canal, whose chief
function is the seizure and connninution of the food. ISIorphologically
considered, however, they are specialized dermal appendages situated in
the buccal cavity, and characterized by the presence of certain calcified
tissue developed from the true derm or corium of the integument,
known as (Icntine. It will be seen from this definition that the term
" tooth," strictly speaking, is limited to those structures of the oral
cavity which alone jwssess such tissue, although it is a recognized fact
that to other epithelial or cuticular structures, found in many inverte-
brate and some few vertebrate forms, the term " tooth " has likewise
been applied.
While they all subserve the same pur})ose, and are therefore analo-
goiis, their chief distinction consists in this—viz. in the latter, so far
as they have been investigated, these organs consist of a corneous or
hornv substance, which is invariably derived from the more superficial
epidermal layer, and is therefore ecdcronic in origin. In the former
a pa})illa arises from the corium, being sunk into a fold or ])it, and
eventually undergoes more or less calcification from its summit down-
ward by a deposition in its substance of lime salts, forming dentine.
The dentine thus formed is a hard, elastic substance, consisting of
closelv-set parallel tubuli, branching as they go, and whose crown may
or mav not be invested with an exceediiigly hard and unyielding sub-
stance derived from the deeper layers of the epidermis, known as enamel.
These are, then, endernnic in origin.
Those of ecderonic source include the so-called teeth of Annulosse,
INIollusca, Insectfe, etc. among tlie invertebrates, as well as the horny
teeth of Ornithorhi/nchus, palatal plates of the Sirenia, and the horny