lamelliform tooth of the Phyllodus hut these teeth being piled one
upon the other, the entire mass presents a succession of strata of
the three substances.
The osseous substance (a, fig. 2, PI. 44), is characterized by. the
large, reticularly anastomosing medullary canals, without radiated cells
in their interspaces, which are peculiar to the structure of the bones
and ossified basis of the teeth in fishes. The dentine (6) consists
of numerous, closed-set calcigerous tubes and the clear uniting
substance; the tubes are characterized by their straight, and parallel
course ; at the middle part of the plate, they are directed vertically to
its plane, and at the margins wrhich are bent down, they incline; so
as to maintain the same relative position to that part of the surface
of the plate; their diameter does not exceed j^ th of an inch;
their subdivision into pencils of smaller tubes takes place nearer
to the enamel than usual.- -I could plainly discern the anastomoses
of these divisions of the calcigerous tubes in some parts of the section.
The enamel c, which, as in the denticles of the Scarus and
many other fishes, closely approximates in structure to the dentine,
exhibits, however, much less parallelism in the course of its component
tubes in the Phyllodus; but these are as numerous and
distinct, though somewhat more minute than those of the true
dentine.
It would seem that in the matrices or pulps of both the enamel
and dentine, the progress of calcification followed the same
law, viz: from the circumference to the centre, or from the surface
to the attached base of the pulp. In specimens of the Scarus preserved
in spirit, and in other fishes, I find that neither surface of the
formative pulps is free, for, while that which may be termed the base
of the enamel-pulp is adherent to the capsule, and while the base of
the dentinal pulp turned in the contrary direction, coheres with the
mucous surface from which it was originally developed, the opposite
surfaces of both pulps firmly adhere to one another. It is at this
surface, however, in each case that the process of calcification commences.
The linear groups of cells being here irregular in their
position, form, by their confluence, tubes as irregularly disposed ;
but as the deposition of the hardening salts proceeds, the tubes
become more regular and parallel. This parallelism has taken place
in the Phyllodus, much sooner in the dentine than in the enamel,
as is the case in the Scarus, and hence the difference of the disposition
of the fine calcigerous tubes, which mainly distinguishes the
texture of the dentine and enamel in fishes.
The fossils here described very clearly exhibit the effects of
attrition and waste at one extremity, and of renovation at the
opposite end of the dental mass; but they likewise show the
same antagonist influences operating at the upper and the lower
surfaces of the mass ; the work of destruction and of reproduction
has proceeded in both the longitudinal and vertical
directions. As the lamelliform teeth at the top of each pile were
worn away, the loss was supplied by new lamellae added to the bottom
of the pile, according to the mode of reproduction described in the
dental system of the Diodon. But as the attrition was greatest at
the anterior extremity of the dental plate, and the power of reproducing
the lamellae in the vertical direction limited, the loss of
entire piles of teeth was supplied by the addition of new piles to the
posterior extremity of the dentigerous plate, according to the mode
of reproduction described in the pharyngeal teeth of the Scarus. In
the Diodon we see illustrated the law of succession of the permanent
to the deciduous teeth in the Mammalia, viz: in the vertical direction,
but the process is much more frequently repeated. In the Scarus, the
coursd of succession of the true molars in the Mammalia, viz : in the
horizontal course was followed. In the Phyllodus, both kinds
of displacement and succession are exemplified ; and the peculiarities
of the Diodon and Scarus were combined, with the same frequent and
uninterrupted repetition of the renovating processes, in a single
dentigerous bone of that extinct species.
SALMONOIDS.
57. Many fishes of the Salmon tribe, like those of the Clupeoid
family, have the superior margin of the mouth formed in a greater or
less degree by the superior maxillary as well as by the intermaxillary
bones, both of which, excepting in the edentulous species, as the
Salmo edentulus,(\) Bloch, are armed with teeth.
(I) The type of the genus Anodus of Spix.