LEPIDOSIREN.
dental plate, but is narrower in the upper one. The figures 1 2
and 3, PI. 59, which give respectively a side view, front view and
the working surfaces of the teeth of the Lepidosiren, will convey a truer
idea than can be given by words, of the form of the maxillary dental
plates The two anterior lobes or divisions of both upper and lower
dental plates are the most produced in the vertical direction, and their
anterior angle is pointed and adapted for piercing. The posterior
divisions are most extended in breadth and least in height, and
terminate in a sharp trenchant edge; the middle lobes present an
intermediate structure.
These teeth, in their paucity, large relative size, and mode of
adaptation to the jaws, resemble the dental plates of the Chimseroid
and some of the extinct Hybodont cartilaginous fishes, as Cochliodus
and Ceratodus Ag.; but they are unlike these in microscopic structure-
approaching in this respect, as will be presently shown, nearer to
the teeth of many osseous dishes. The maxillary armour of the Lepidosiren
surpasses any known dental apparatus in the class of fishes in
the modification of the working surface, by which a single tooth is
at once adapted for piercing, cutting and crushing the alimentary
substances. The strength of the jaws of the Lepidosiren and
the bulk of the muscles which work them are proportionate to
the size and nature of the maxillary dental plates.
In PL 59, fig. 4, is given a reduced representation of a magnified
view of a vertical section of a lobe of the lower dental plate of the Lepidosiren.
It consists, as in the cod and sphyrana, of a central mass of
coarse osseous substance, traversed by large and nearly parallel
medullary canals, and an external sheath of very hard enamel-like
dentine. The medullary canals are continued from a coarse reticulation
of similar, but wider canals, in the substance of the supporting
bone, and advance forwards, nearly parallel with each other, and
with the plane of the upper surface of the tooth; they anastomose
together by short, curved, transverse canals, which intercept spaces
increasing in length as the canals recede from the osseous basis
The canals themselves diminish in size in the same ratio, and when
they have arrived near the dense outer layer, their divisions and
inosculations become again more frequent, the peripheral loops forming
a well marked line of demarcation between the coarse-tubed
and the fine-tubed dentine. The interspaces of the medullary canals
are occupied by a clear substance and by moss-like reticulations of
fine calcigerous tubes, which appear to be more sparing in number
than in the teeth of the sphyraena or shark. The calcigerous tubes
of the external dentine run nearly parallel to each other, and vertically
to the external surface of the dental plate through about two thirds
of the thickness of the external hard substance ; they then bend and
cross each other in a manner very similar to those of the external
layer of dentine in the teeth of the Lepidotus, Phyllodus, &c.
In the process of attrition this external dense substance is worn
away from the upper surface of the dental processes in the lower jaw,
exposing the softer osseous, or medullary substance of the tooth ; in
this state the dental plate offers an analogy to the incisors of the
rodents, a posterior softer substance being sheathed by an anterior,
denser layer ; and an external sharp edge is similarly kept up by the
unequal wearing away of the two substances. The progressive waste
at the upper surface of the dental plate would appear to be met
by a corresponding addition of new material to its lower part.
In the structure here presented to our observation we have a
condition of the dentine which has hitherto been met with only in the
class of fishes; and the form, the extent, and the continuity
of the dental armature excepted, this part of the Lepidosiren
closely corresponds with that particular modification of the dental
structure which we have seen to be most eminently characteristic
of the class of fishes. The Sauroid character, for example, which
as in the Lepidosteus, pervades the air-breathing organs, and
which, as in the Polypterus, is traceable in the intestinal canal of
the Lepidosiren, is not at all manifested by the dental system: and
neither in the modified bone which forms the basis of the tooth, nor
in that coarser bone of which the jaw is composed, is there the
slightest trace of radiated cells or corpuscules; the other parts of the
skeleton exhibit a similar ichthyic condition.
The test of the affinities of the present paradoxical genus, afforded
by the microscopic examination of the teeth, gives additional confirmation
to the views which I have already maintained from argu