sition of a series of concentric lamellae in proportion to its term
of use.
The ramification of the calcigerous tubes in this tooth, presents
the same general character as those of the Acrodus, hut they are
shorter ; and each medullary canal, with the radiating series of tubes,
is seen in the transverse section to be separated from the contiguous
one, by the regular boundary lines above-mentioned, which lines distinguish
the teeth of the Myliobates from those of the Acrodus, Psam-
modus, Cestracion, or any of the shark-tribe. The rostral teeth of
the saw-fish, and those of the Orycteropus, among mammalia, present
the nearest resemblance in their intimate structure to the teeth of the
Myliobates. Plate 27 gives a correct general idea of the structure of
the present tooth as displayed by a transverse horizontal section near
the root.
The teeth of the Myliobates, like those of the rest of the plagio-
stomes, are successively formed at the posterior part of the tesselated
series in proportion as they are worn away in front. A series of minute
and closely aggregated papilliform matrices rise from the mucous
membrane behind the teeth, and are covered by a fold of the same
membrane which is reflected forward so as to conceal the pulps and
the last formed teeth. The papilliform pulps are ossified by the deposition
of the calcareous salts in the peripheral cells and radiating tubes,
hut the medullary or central canal of each pulp continues to retain its
organizer and vascular contents till the whole of the compound tooth
is completed; the calcified wall of the medullary canal is then thickened,
and the area diminished by the successive formation of concentric
laminae of osseous matter. In Zygobates, the subgenus in which I
have studied the development of the teeth of this family of rays, the
middle tooth of each transverse series is first developed ; the formative
papillae of the two broad lateral teeth begin to rise first at the
mesial and anterior parts of the tooth, and from these points succeed
each other to the posterior and outer sides | that facet which is adapted
to the posterior-lateral facet of the median tooth is first completed,
and at this period the broad lateral tooth presents a trigonal instead
of a hexagonal contour, the whole length or vertical diameter of the
mesial side of the tooth is completed before the formation of the outer
side of the tooth is begun. These facts are strikingly opposed to the
theory of the formation of a tooth by the transudation of layers from
the superficies of a pre-existing glandular pulp.
As the teeth of the Myliobates are gradually carried forwards into
action by the direction of growth of their basis of support, the areae of
the medullary canals become progressively diminished, as in bone, by
osseous deposition in concentric layers, and are thus finally consolidated
in the anterior teeth.
CESTRACIONTS.
18. The dental characters of this family of cartilaginous fishes, are
»chiefly manifested in a form of tooth, better adapted for crushing or
■ comminuting alimentary substances which offer only passive resistance,
«than for piercing, cutting and lacerating a living prey ; and this less
I formidable character of the maxillary armour is compensated, in gene-
Iral, by the development of two formidable spines upon the back of
■ the fish. In most of the species, the teeth also vary in form and
»size in the same individual to a greater degree than in the sharks ;
land in all the cestracionts, their substance is traversed by medullary
I «canals whose systems of calcigerous tubes are not separated by well
Idefined boundaries, as in the Myliobates.
Of the numerous singular forms of this tribe of cartilagi-
| inous fishes that once peopled the seas of the northern hemi-
[ «sphere, and which have left their less perishable remains in the
«secondary strata of the present dry land, all have now disappeared,
jaand the sole existing representative is the genus Cestracion,
jof which the most common species is met with in the Australian
■ Seas: a second species has been indicated which frequents the
|southern coasts of China. The ancient fossils above alluded to
| fwould have been scarcely intelligible unless the key to their nature
lhad been afforded by the teeth and spines of the existing Cestracion.
In the Port Jackson shark, (Cestracion Phillippii), the jaws form
k greater proportion of the skull than in any other existing cartilaginous
and plagiostomous fish;(1) they are also more elongated and
fhrected more horizontally forwards, thus approaching nearer to the
(1) PI. 10, fig. 1.
£