MOSASAURUS.
106. One gigantic extinct species of Saurian Reptile has been
found to agree with many of the species above cited in the Lacertian,
Iguanian, Anolian and Scincoid families of existing Saurians in having
the pterygoid bones armed with teeth : but the maxillary teeth combine
the pleodont with the acrodont characters ; and the skeleton
indicates a special adaptation for swimming and a marine life.
The true affinities of the Mosasaur, the extinct reptile in question,
which was at least twenty-four feet in length, and the remains
of which characterize the chalk-formations, were first determined by
Cuvier, who places it in the Lacertine group of Saurians between the
Iguanee and Monitors. Its dentition exhibits in an eminent degree
the acrodont character ; the teeth being supported on expanded conical
bases anchylosed to the summit of the alveolar ridge of the jaws:
no existing Saurian exactly parallels this mode of attachment of the
teeth, either in regard to the breadth of the alveolar border, or in
the relative size of the osseous cones to the teeth which they support.
A shallow socket is left where the tooth and its supporting base are
shed. The form of the teeth is likewise different from that hitherto
observed in any existing Saurian : they are pyramidal, with the outer
side nearly plane, or slightly convex, and separated by two sharp
ridges from the remaining surface of the tooth which forms a halfcone
; the transverse section of the tooth near its attachment to the
osseous base presenting the contour given at PL 72, fig. 5. All the
teeth are slightly recurved and their peripheral surface is smooth. The
teeth are implanted upon the intermaxillary, maxillary and preman-
dibular bones ; a series of similarly shaped but much smaller teeth
are placed upon the pterygoid bones. The superior maxillary bone
in the great cranium preserved in the Parisian Museum—the most
celebrated fossil of the present species—contained eleven teeth :
Cuvier calculates that the intermaxillary bone may have contained
three teeth ; meaning probably three on each side : the premandibular
element of the lower jaw supported fourteen teeth : the number of the
teeth thus approximating to that which characterizes the Varanus Nibticus.
They are arranged in a pretty close and regular series. There
appear to have been eight teeth on each pterygoid bone.
In the mode and place of development of the successional teeth,
the Mosasaurus resembles the Iguana} and most other Lacertians.
In the great cranium above mentioned germs of new teeth in various
stages of growth are lodged in hollows of corresponding degrees of
depth on the inner side of the bases of the adherent teeth, and have
evidently owed the commencement of their formation to the mucous
membrane which originally covered those supporting cones of the
teeth in place. The attention of Camper was particularly arrested by
the observation of this fact, which appeared the more singular to him
as this mode of dental succession, which is common in reptiles and
osseous fishes, was not then known.
“ The dentition is so singular,” he says, “ in these fossil jaws
that it deserves a particular description. A small secondary tooth is
formed complete with its enamel and solid root in the osseous substance
of the temporary tooth: in the progress of its growth it seems
gradually to form a cavity of corresponding size in the osseous root
of the primitive tooth; but it is impossible for me to decide what next
befalls it, or in what manner it is shed.”
The crown of the tooth consists of a body of simple and firm
dentine, invested with a moderately thick coat of enamel; the expanded
base is composed of a more irregular mass of dentine which,
by its progressive subdivision into vertical columnar processes,
assumes a structure resembling that of true bone | this part is covered
with a layer of cement, which is continued as an extremely thin coat
upon the enamel.
The pulp-cavity generally remain's open at the middle of the
base of the crown of the tooth ; irregular processes of the cavity
extend, as medullary canals into the conical base of the tooth; but no
processes of tbe pulp-cavity are continued, as in the Iguanodon, into
the substance of the coronal dentine. This substance consists, as in the
Crocodile, of fine and close set calcigerous tubes, arranged according
to the usual law; and much resembling that of the tooth of the vara-
nian Monitor figured in PL 67 : the calcigerous tubes have a diameter
°f le.Lth of an inch : with interspaces equalling about four of these dias
2