T h e S k e l e t o n o p t h e G a n g e t ic D o l p h in .
Cuvier1 was the first anatomist to point out the great maxillary crests arching
upwards and inwards over the blow-hole; the extraordinarily laterally compressed
upper jaw; the long symphysis of the lower jaw; the remarkable development of the
temporal fossa; the great size of the zygomatic process of the temporal; its application
to the post-orbital process of the frontal; the small size of the orbits; the great
thickness of the crests between the basilar and lateral parts of the occipital hone,
which inwardly confine the vault under which the ear is situated; the union of the
bulla tympani to the petrous, which latter is firmly wedged in between the temporal
and surrounding parts of the occipital bone; the free character and great length for a
Cetacean of the cervical vertebrae and the development in them of the lower transverse
processes as in whale-bone whales; the large character of the scapula, the absence of
a supra-spinous fossa, the presence of a large acromion and the rudiment of a
coracoid. I t was on such facts as these that Cuvier urged that the dolphin of the
Ganges was a distinct genus remarkably different from any other dolphin, although
he did not assign to it any distinct position among the toothed whales.
The next anatomist to throw a flood of light on its structure was Eschricht,
in his well-known Memoir, in which he followed Cuvier’s description step by step,
adding to it and correcting some of Cuvier’s observations. His most important
contribution to the anatomy of the skull was his explanation of the structure of
the temporal fossa and his indication of the probable site of the palatines in the
nasal chamber, and his account of the pterygoids. The whole of his Memoir,
however, is replete with information regarding the structure of the dolphin and is
rich in comparisons of it with other Cetaceans, especially Syperoodon, which he
considered to be its nearest ally, although in some respects it was related to the
Whitefish and to the allied great toothed whales, and in' other regards to Inia,
the fluviatile dolphin of the Amazon, whilst he held it to be quite isolated by other
features of its structure.
By its maxillary crests the Gangetic dolphin in Eschricht’s opinion was more
closely allied to JHyperoodon than to Micropteron or any other fossil Cetacean. He
admitted that the analogy in the structure of the cranium with the Syperoodon,
and partly with the cachalot, is altogether lost as regards the rest of the skeleton,
in which there are striking differences between the two forms.
To the late Dr. Gray2 belongs the credit of being the first to bring out prominently
the affinity manifested by Platanista to Inia; but although Eschricht
allowed that this attempt by Dr. Gray to express the relationship of the animal
was equally successful with that by Wagner who placed it between Inia and
Micropteron, yet he held that d’Orbigny’s incomplete account and figure of Inia did
not reveal any of the peculiarities of the cranium of the Gangetic dolphin, and that
1 Ossemens Eossiles, vol. viii, PI. 1836, p. 88, et seq. et p. 128.
9 Voyage, Erebus and Terror, 1846, p. 46.
although no one could hesitate to separate it as a distinct genus and to regard it
as probably not far removed from Inia, he considered that the two could hardly
be placed under one common group, the Plata/nistina, under which Gray brought
together the genera Platanista, Ima and Pontoporia, and which genera he has
more recently1 elevated into as many families.
The osteological features, however, of Ima were very imperfectly known
when Eschricht wrote, but now through the labours of Mower2 the structure of its
skeleton has been well ascertained. Professor Gervais3 in 1855 grouped Platamsta,
Inia and Pontoporia to form one of the five tribes (Plata/nisti/ns), into which he
divided the Selphinidoe, the last but one of his four primary divisions of the
Cetacea.4
Professor Elower has pointed out that Inia, Pontoporia and Platamsta agree in
the great development of the temporal fossa, which, however, is most marked in
Platamsta, and in this striking feature of the skull the last differs from all the other
dolphins. In Platcmista and Pontoporia the zygomatic process of the temporal
abuts against the back of the orbital process of the frontal, so that there is no postorbital
process, bjxt in Ima this process is separated from the post-orbital process
therein developed by a considerable interval. In all of these forms the zygomatic
process of the temporal has a great family likeness, and is distinguished from the
corresponding processes in other dolphins by its markedly stronger character, which
relates to the great development of the temporal fossa. In the skull of Platamsta
which formed the subject of Professor Elower’s observations, he mentions that it
differed from that of Inia in the construction of the temporal fossa, namely, in this,
that the frontal and squamous were not separated from each other by the pointed
extremity of that portion of the parietal which forms the side of the fossa, whereas
in Ima the squamosal was shut off from articulating with the frontal, and in
Pontoporia the parietal prevents the union of the frontal and squamosal, and the
pterygoid articulates with all of these bones. In Platanista, however, there would
appear to be considerable individual diversity as to the extent to which the pterygoids
separate the squamous from the frontal, which is certainly occasionally the case in the
skulls which have come under my observation, although the arrangement described
by Professor Elower is what is generally present. This variation is so modified in
different individuals that it cannot be ascribed to any other cause than an inherent
tendency in these bones slightly to alter their forms.
Professor Mower has supplemented Eschricht’s observations on the construction
of the hard palate and has proved the correctness of the latter’s suggestion that
the palatine would be found in the nasal tube, and that the bones appearing externally
on the palate and entering into the temporal fossa were the pterygoids, and
in these features Platamsta is unique among the Cetacea. Inia itself, which
1 Cat. Seals and Whales, B. M., p. 87, et seq.
2 Trans. Zool. Soo., 1869, vol. vi.
3 Hist. Nat. des Mammif., 1855, p. 321.
I t is a subject of regret to me that I have not had the privilege to consult the 13 Livr. of Van Beneden and
Gervais’ splendid work 1 Ostéographie des Cétacés.’
p 3