of conversion, not of excretion. As the teeth advance into use, the
organized pulp of each medullary canal becomes consolidated by the
formation of concentric osseous layers at its circumference, and by an
irregular calcareous deposition in its centre. The new teeth are
carried forward and outward by the same rotatory movement of the
membranes supporting them, as in the ordinary Squaloids.
19. Acrodus„—Among the extinct Plagiostomes recognizable by
teeth or spines, the genus Acrodus, according to M. Agassiz, presents
the closest affinity with the existing Cestracion. The teeth of the
Acrodus, (PI. 14, fig. 1 ,) are of an oblong form with a convex or
slightly conical crown, supported on a coarse osseous base, in the
form of a parallelogram, obliquely truncated at its inner side. The crown
swells out beyond the base, and is expanded in the middle, and contracted,
generally unequally, towards the extremities ; it is traversed
by fine ridges, which diverge from one which runs along the middle of
the long diameter of the tooth. Portions-of the jaw of the Acrodus have
been discovered which show that these teeth were arranged, as in
Cestracion in oblique rows, with, at least, seven teeth in a row. Those
at the anterior part of the jaws were short, with the middle of the
crown raised into an obtuse cone; the posterior teeth were more
elongated and depressed.
Professor Agassiz has remarked that ancient writers on Natural
History have mistaken the teeth of the Acrodus for fossil insects or
worms : by the quarry men of this country they are commonly called
petrified leeches: by some naturalists, they have been regarded as
auditory ossicles or otolithes of fishes ; but, independently of more
obvious evidence, a single inspection of a transparent slice of one of
these fossils in an adequate magnifying power, would suffice to demonstrate
their real nature, (PI. 14 and 15).
The teeth of the Acrodus nobilis are composed of two substances,
viz : a thin external almost colourless layer, which represents the
enamel, and an amber-coloured coarser dentine composing the body
of the tooth, and continuous with and passing into its coarse
cellular bony basis and support. Microscopic sections of this
tooth afford the most beautiful appearances, and, perhaps, the most
instructive illustration of the relation of dentine to bone. The body
of the tooth consists of groups of short, branched and frequently
irregularly wavy medullary canals imbedded in a clear matrix.
These canals are surrounded by concentric layers, and closely
resemble the Haversian canals in true bone. The calcigerous
tubes, which radiate from the medullary canals, have a graceful
undulatory course, and are much branched; but towards the
periphery of the tooth, the ramified tubes are all directed, as in ordinary
dentine, at right angles to the superficies, and thus constitute a
regular layer of hollow columns disposed so as to offer the greatest
resistance to external pressure. This layer is equal in thickness to
about one-fifteenth part of the vertical diameter of the thickest part
of theT htoeo tfhi.n est or terminal branches of this peripheral layer ot,
tubes, I have traced in various places into what, at first sight, appears
to be the enamel. Under a magnifying power of 400 diameters,.however
this outermost layer is seen to be composed of extremely minute
tubes -w th of a line in diameter (PL 16, fig. ■ ; they are branched
like the. coarser tubes of the body of the tooth ; irregularly wavy in
their course, having a general tendency to an arrangement at right
angles to the superficies, but inextricably interwoven, and connected
anastomotically together, so as to require a strong light to penetrate
even the thinnest section, and render their structure and arrangement
visible The continuation of these finer superficial tubes with the
coarser tubes of the body of the tooth, is best observed by slightly
changing the focus of the glass with which they are viewed, which
brings the transitional tubes at different depths in the section into
view. In some parts of the section, a medullary or Haversian canal
is displayed longitudinally ; and the parallel lines of the surrounding
concentric strata on each side are exhibited. The canal maintains a
general uniform diameter, but slightly dilates where it divides or
sends off a cross branch to communicate with the adjoining canals.
These canals commence from the large cells of the bony base, and
pass into the substance of the tooth towards its periphery; they
communicate by transverse canals, but all ultimately terminate in
bundles of the wavy ramified calcigerous tubes of the body of the