Platanist in which the ribs are attached direct to the bodies of the vertebrae in
the sixth, seventh, eighth and ninth vertebrae, owing to the entire absence of any
intervening process or projection from the bodies of these vertebrae, and owing
perhaps to the imperfectly developed character of the transverse processes, the latter
appear suddenly to cease, whereas in the adult the transverse processes appear gradually
to subside on to the bodies of the vertebrae and to exist as tolerably well
marked short processes even on the eighth and ninth vertebrae on which in the
young there is no trace whatever of such a process, the structures afterwards developed
in these as in the seventh also being only outgrowths of the bodies of the
vertebrae. The process bearing the tenth rib is entirely different, and is a distinct
structure developed quite independently from a centre of its own. These processes
are apparently developed from behind forwards, as in a young skeleton those of the
caudal and posterior portion of the lumbar region are firmly united to their vertebrae,
while those bearing ribs and a few behind them are not so united to their segments.
Platanista in having eight lumbar vertebrae more resembles Pontoporia than
Inia, as the former, according to Burmeister, has six or seven1 lumbar vertebrae,
whereas Inia is stated by Elower to have only three or four, which is a marked
difference from what prevails in the ordinary JDelphi/nidce, in which the lumbar region
has great extension.
Professor Elower remarks that “ nothing can be more dissimilar than the lumbo-
caudal region of the special column in Inia and Platanista,” and from Burmeister’s
researches on the anatomy of Pontoporia we may add, between this dolphin and
the Gangetic form.
In Platanista, as I point out hereafter, the sternal ribs, although cartilaginous
in youth, become ossified with adult life, but always with a small portion of cartilage
or ‘ intermediate rib,’ as in the Monotremes, intervening between the end of the
i ib and the ossified sternal rib, whereas in Inia the sternal ribs are always cartilaginous.
Elower in grouping Platanista, Inia and Pontoporia with Physeter, Hyperoodon
and the Ziphioids did so under the impression that the absence of ossified sternal ribs
was a character common to them as a group. But now, in addition to the sternal
ribs of JPlatcmista being known to be ossified in adult life, Burmeister2 has shown
that the sternal ribs of JPontoporia are ossified, as in marine dolphins generally, to
which it is allied.
Such facts have an important bearing, as Professor Elower who is facile princeps
in this department of anatomy, when seeking for some starting-point or basis for a
primary division of the toothed whales, Odontoceti, selected this presence or absence of
ossified sternal ribs as “ a character derived from a part of the organization apparently
less liable to adaptive modifications than the teeth or fins.” I t would now appear,
however, that this character has not that reliability which it was thought might be
attached to it. The facts already adduced, and those yet to follow in this contribution
to the anatomy of JPlatcmista, seem to me to confirm the just appreciation which
1 Proc. Zool. Soc., 1867, p. 485.
3 Loc. § j|| p. 412.
Cuvier, Eschricht and Elower had formed of its anomalous character, and to establish
the position assigned to it by Dr. Gray as a distinct sub-family of the toothed
whales, including the genus Inia, and through Pontoporia connected with the true
JDelphi/nidce.
Supra-occipital (PI. XL, fig. 2).—In the skull (10'75 inches long) of a young
male caught in August, and therefore probably only a few months old, this bone is
quite distinct from the exoccipital, but confluent at its upper angles with the parietals.
The obscure external occipital crest is prolonged downwards, towards the margin of
the foramen magnum, terminating in the incision or cleft (fig. 2, cl). There is a
slight concavity on the sides of the upper half of the ridge, but the lower half of
this surface of the bone is somewhat convex. The upper two-thirds of the internal
surface are deeply concave, the lower third almost flat. The former portion is traversed
by a prominent sharp vertical ridge, with no trace of a groove for the
longitudinal sinus, but the transverse ridge shows indications of grooving for the
lateral sinus. The sides of the ridge are marked by numerous nutrient foramina,
and the fossæ themselves by long fine meningeal grooves. The cerebellar fossa is
nearly flat in its .upper half, but concave above the margin of the foramen magnum.
The parietal margin of the bone equals the length of the exoccipital border.
In another and older male skull; measuring 14 inches in length, the parietal and
frontal edges of the supra-occipital are slightly prolonged backwards, with an inward
curve in the case of the former and backward curve in the case of the latter, below
which there is a considerable concavity. The bone, however, is' convex from the
parietal to the margin of the foramen magnum, and the feebly-marked vertical ridge
has a diminutive tuberosity. The incision at the foramen magnum is closed at the
border of the orifice, but an elongated imperfection still exists above this. The foramen
magnum preserves the same form as in the foetal skull. The internal vertical
ridge is very sharp.
In a male skull, adult or nearly so, the occipital region has ' undergone a
remarkable change of form, which is also characteristic of the female. I t is oblong
and deeply concave between the parietals. This is due to the development of a
prominent lambdoida! ridge, turning outwards and downwards in the position of
thé upper angle of the exoccipital to the base of the zygoma, and to an equally
marked sagittal crest. In one adolescent adult, there is a rough nodular tuberosity
but no ridge, while in a female of considerable adolescence, so to speak, the occipito-
interparietal region is narrower and deeper than in males of apparently nearly the
same age, and there is neither a trace of a tuberosity nor of a vertical ridge ; yet, in
another example, adult and of the same size, the tuberosity alone is developed. In
adult skulls the incision at the foramen magnum disappears.
JExoccvpitals.—In the foetal skull, the line of separation between these bones
and the basi-occipital cannot be detected, the exoccipitals being only ossified on a
line with the inferior extremities of the facets for the atlas, but anterior to that and
between the facets there is a considerable cartilaginous area. Before the facets they
are rather deeply concave, but their external surfaces are rather convex, especially