ance with the principle of arrangement of the Physiological department
of the Hunterian Museum, the Dental System is here traced
from its more simple to its more complex conditions. But this
progress is partially subordinated to the limits of zoological arrangement.
For, although the tooth of a Myliobates or a Labyrinthodon be,
in structure, more complex than many Mammalian teeth, yet this
complexity is associated with other characters, such as mode of
attachment, frequent shedding and renewal, &c., which indicate an
essentially inferior grade, and connect them, respectively, in closer
natural relationship with the more simple teeth of other species of
Fishes and Reptiles. A distinct Part, or division of the Work is,
therefore, appropriated to the Dental System of each of the three
great Classes of Vertebrated Animals which possess teeth.
In the Mammalian series the course of progressive complication
of the teeth is closely followed, irrespective of the general grade of
organization of the species, and the Human dentition falls, accordingly,
into the middle of the series. Guided by the evidence of the
teeth I have sometimes deviated from the accepted Zoological systems,
as will he seen in the last Chapter, devoted to the complex dentition
of the great Family of Hoofed herbivorous quadrupeds,
and especially in the value there assigned to the Ruminant modification
of the Ungulate type.
In each Class, the chief characters of the teeth of the
extinct species are described in connection with those of the
allied existing forms. For so vast is now the extent, and so rapid
the progress of Palaeontology, and so important are the links in
the chain of Being thus recovered, that no treatise on the Comparative
Anatomy of the enduring parts of animals can fulfil its
expected purpose, if it he restricted to the description of such parts
in existing species alone.
With regard to the Teeth, some of the most interesting and
extraordinary modifications were peculiar to species that have long
since passed away from the stage of animated existence; and,
indeed, no comprehensive view could he obtained of the dental
tissues without a knowledge of those intermediate conditions which
they present in fossil teeth. I need only refer to the Acrodus,* the
Sphcerodus,\ the Saurocephalus,\ the Dendrodus,§ the Labyrinthodon,||
the Iguanodon,% and the Megatherium,** in illustration of the value
of Fossil remains, and of the microscope, as an instrument in the
determination of their nature and affinities.
This important application of the microscope has, however,
its limits, and from the difficulty of testing the described results by
repetition of the observations, and from other causes, it is liable to
be abused.
A knowledge of the structure of the entire fossil tooth should
be obtained by longitudinal and transverse sections ; at least by a
transverse section through the entire thickness of the crown. With
this knowledge a fragment of the tooth of the same species may
afterwards he determined: and also in many cases those of other
species of the same genus, or natural family: without it, great
mistakes may he committed. If, for example, a microscopic observer
had begun his examination of an Iguanodori’s tooth by a slice of a
fragment from the outer half of the crown, and had afterwards
examined another fragment of the tooth of the same species taken from
the inner half, he would, most probably have referred such fragments
to two very distinct species of animals. The requisite knowledge of
the characteristic combination of one lateral moiety of dentine, and
another of vaso-dentine in the same tooth, pre-supposes the examination
of a section of an entire specimen. In like manner, to pronounce
on the generic and specific distinctions of fossil Proboscidians
from the characters of portions chipped off the exterior of their tusks,
is an abuse of the microscope, and betrays an ignorance of the mode
and limits of its application.ff
One consequence of an attempt, like the present, to determine
* PI. 14. f PI. 32. t PI. 55.
§ PI. 62 B. The teeth of this extinct Fish afford a beautiful example of the unexpected
application of microscopic characters of dental tissue in the determination of an important
geological problem.—See Appendix to Mr. Murchison’s ‘ Geology of Russia,’ p. 635.
|| PI. 64 A. IT PI. 71. ** PI. 84.
f t Thus, out of portions of tusks of young and old individuals of the Mastodon giganleus,
the genera Missourium, Tetracaulodon, and their different species, have been attempted to be
established.—See Geological Proceedings, June 29, 1842.