presents a close relationship to this form, differs widely from it in these details, as
does also Pontoporia.
As pointed out by Cuvier, the petrotympanic is not merely suspended by means
of fibrous tissue, but is looked into position by a prooess of the mastoid, whereas in
J im®, Elower describes these bones as lqosely attached to the cranium, and from
their absence in the skull of, Pontoporia he concludes that their attachment must be
much the same as in Inia. In Platamsta, on the other hand, the Eustaohiam
extremity of the tympanic is long and. tubular. In Inia this feature is much less pronounced,
and the bone partakes more of the oharaoter that distinguishes it in
Pelphinus.
The resemblance of the lower jaw to that of the Cachalot has been forcibly
pointed out by Cuvier, Esohrioht and Elower; and the latter speaks of the lower
jaw of Inia as a miniature representation of the same jaw, while the mandible of
Pontoporia is intermediate between P latanista and Inia.
The teeth of this dolphin are very different from the firmly implanted short
rugose-crowned teeth of Inia and are entirely destitute of any lobular character, and
they are markedly distinct from'the teeth of Pontoporia, as figured by Elower1 and
Burmeister.a
Comparing the vertebral columns of Platmista, Ima and Pmtoporia, great
differences, besides the fewer vertebra! in the two latter as compared with the
former, are found to obtain. The most marked feature of Platamsta is the, so
to speak, enormously developed oblique processes by means of which the individual
vertebra! from the fifth dorsal backwards for a long way are interlocked with strongly
forwardly directed spinous and well-developed transverse processes in the lumbar
and anterior portion of the caudal region. In Inia there is only a faint trace of
metapophyses, and all the interlocking so characteristic of Platmmta is absent,
but while in Pontoporia metapophyses are present in the dorsal, and partly in the
lumbar region, its remarkably shaped transverse processes are very different from
those of the Platanist.
As is well known, one of the most distinguishing characters of the vertebral
column of the Platanist and by which it is separated from all Cetacea except Imat
is the unusual length of its neck, which is nearly as well developed as in many quadrupeds.
The cervical vertebrae are marked by considerable transverse extension
and by depression, and in these characters they differ from the vertebrae of Ima,
while, however, as in Inia and Pontoporia, they are all free, but.the neck of this
la st m e n t i o n e d C e t a c e a n is considerably shorter than in J»i®.
In the atlas, as in Pontoporia,1 the spinous process is feebly marked, and thereis
only one long and strong transverse process, but in Inia there is a well-developed
spinous process and the rounded rudiments of two transverse processes, upper and
lower. In the Susu the under surface of this bone has a long prominent bifid process
1 Loc. cit. ' y . ... • 2 Annales de Mus. Pub. de Buenos Aires, 1869, PI. xxvu, figs. 2a and 3a.
* Burmeister: loo. cit.
more strongly marked in Inia. A distinct odontoid occurs in Ilatanista, but the
equivalent of this process is only feebly shown in Inia, and the spinous process is
broad and bifid, while in the former it is powerful with considerable antero-posterior
extension. In Platamsta the union of the upper and lower transverse processes
occasionally takes place, but it is usually confined to one side, and I have observed
it in the fourth vertebra only, as this would appear to be the first segment in which
the two processes first exist, and in which a perfect ring is sometimes formed, as in
the third, fourth and fifth vertebrae of Pontoporia} This, however, would appear
to be perfectly distinct from the adventitious perforation of the upper transverse
process of the third vertebra noticed by Eschricht, but to be identical with the well-
marked canal occasionally formed in Inia and Pontoporia in the same vertebra
by the union of the upper and lower transverse processes as described by Elower
and Burmeister. Platmista differs from Inia in the- great development of the lower
transverse process of the sixth vertebra, and in the absence, as a rule, of this process
in the seventh cervical, whereas in the latter vertebra it usually exists in Inia in a
rudimentary condition, the spinous process of this vertebra being very feebly developed
in Inia and^Pontoporia, but strongly in Platamsta.
In Platamsta, the first dorsal has two facets on either side of the body for the
articulation of the heads of the first and second ribs, but on the second and succeeding
vertebrae to the sixth, only one facet occurs for the head of the rib, and it is
situated on the posterior margin of the body obliquely behind the articulation on
the transverse process for the tubercle of the rib anterior to it. There is a distinct
approximation of the facet for the heads and the tubercles of the ribs, so much so
that the facet for the head of the seventh rib on the sixth vertebra is nearly on
the same level as the articulation for the tubercle of the same rib on the seventh
vertebra. Oh the eighth the head and tubercle have practically coalesced, and it
is this yertebra in the young animal in which the rib is first seen to be distinctly
applied to the side of the body of the vertebra without the intervention of any
process.
I t will be seen from this description that the arrangement of the ribs is much
the same as in Inia, and as in Pontoporia as described and figured by Burmeister.
In Inia, the head of the rib is more attached to the intervertebral substance before
its own vertebra, so that in the fourth and fifth vertebrae the articulation for the
head of the rib1 instead of being on the posterior margin of the vertebra before its
own is on the anterior margin of the segment to which the tubercle of the rib is
attached. Erom the eighth vertebra backwards each rib in Platmista is only
attached to its own vertebra, whereas in Inia this takes place in the sixth, the
coalescence of the articular surf aces for the head and tubercle in Inia, as in Platamsta,
occurring on the eighth vertebra.
The difference between Hyperoodon and Physeter and Inia lies chiefly in the
circumstance that in the two former the upper transverse processes appear suddenly
to cease, and the rib retains its connection with the body only; but in the immature
1 Burmeister, PI. xxvi, figs; 6-8.