typifies that of the teeth in all the Scomberoid fishes. It consists of
a thin external coating of fine enamel-like dentine, and a body of
coarse dentine. The most successfully injected vascular membrane
could not surpass in the number and delicacy of its ramified capillaries
the arrangement of the medullary canals, which pervade the whole
body of the tooth in this fish. The smallest marginal or peripheral
divisions of the medullary canals anastomose in curved loops whose
convexity is turned towards the superficies of the tooth; the branches,
which, by their terminal division, constitute those loops, successively
anastomose, as they widen and slightly converge towards the central
part of the tooth.(1) Here the main medullary tubes run nearly
parallel to each other, with interspaces equalling from four to six
times their own diameter. In their progress towards the base of
the tooth, they give off numerous branches which subdivide and
anastomose in the interspaces of the main canals. These canals, in
the fully formed anchylosed teeth, communicate directly with the
medullary canals of the jaw-bones, which run at right angles with those
of the teeth. (2j The reticulate medullary canals every where send off
the much more minute calcigerous tubes, which quickly subdivide into
tufts or pencils ; these soon begin to bend and interlace together and
fill the meshes of the medullary reticulations by their inextricable anastomoses.
The peripheral loops of the medullary canals, in like manner,
give off pencils of fine calcigerous tubes ; but these maintain, as do the
tubes of ordinary dentine, a more regular and parallel course ; they
penetrate, and constitute with the clear uniting substance, the external
hard and white coating of the tooth ; the first two-thirds of their
course is at right angles to the plane of the surface towards which
they are directed ; their finer ramifications then begin to bend from
side to side, and terminate in reticular inosculations of extreme
minuteness, which are finally lost in the clear dense substance
of the outer layer.
Sphyroenodus.S-The same general distribution of the medullary
(1) A diminished view of the structure, as seen in a longitudinal section of the apex of
the tooth, is given at fig. 2, pi. 53.
(2) The solid tissue of the maxillary bones presents a close-set parallel arrangement of
undulating calcigerous tubes, and nearly resembles the texture of the dentine in the higher
mammiferous teeth. There are no Purkingian corpuscles or radiated cells in this bone.
and calcigerous tubes pervades the whole substance of the tooth in
many, and probably in all, of the Scomberoid Fishes, and not only in
those of the existing epoch, but the same beautiful and complicated
structure may be discovered with equal distinctness in the fossil teeth
of species and genera which are now extinct. At Plate 54 is given
a view of the reticulate disposition of the medullary tubes, as seen
by a low power, in a transparent longitudinal section of the fossil
tooth of a large Scomberoid Fish, from the eocene formation called
the London clay, at the Isle of Sheppey. The tooth, which I formerly
described under the name of Dictyodus,{ 1) in reference to the arrangement
of the-medullary tubes, belongs to the extinct genus of
fishes called by M. Agassiz, Sphyrcenodus, from its affinity to the
existing genus, Sphyrcsna, the largest species of which it must have
equalled in size.
The natural form of this tooth, which is from the lower jaw, is
conical and subcompressed, but relatively thicker than any tooth of the
Sphyrsena : its base is broad, but implanted as in that genus, in a deep
socket, to the bottom of which it is anchylosed. The main body of the
tooth consists of a coarse dentine, incased by a thin layer of fine and
hard enamel-like dentine. The coarse dentine is pervaded by an assemblage
of medullary canals, directly continued from the large irregular
medullary sinuses and cells at the anchylosed base, and thence
proceeding in a general parallel course towards the apex, sparingly
dichotomizing, and gradually diminishing as they proceed : they are
separated by interspaces, which are generally equal to three or four
of their own diameters, and send off, throughout their course, short
transverse branches, which anastomose and intercept in the interspaces
of the longitudinal tubes, quadrangular, sub-elliptical, pentagonal,
or hexagonal spaces, generally elongated in the axis of the tooth,
but becoming shorter as they approach the periphery, especially at
the apex, where the structure of the tooth resembles an irregular lace-
work. In this fossil I have been able to detect the fine calcigerous
tubes only in the hard peripheral layer of dentine, into which they
pass directly from the nearest medullary canals, dividing and subdividing
at acute angles, but with a general direction vertical to the
(1) Trans. Brit. Association, 1838, p. 112.
K