94 MR. W. H. FLOWER ON THE OSTEOLOGY OF 
but as the adult and apparently perfect skull from Ega, in the British Museum, shows a 
precisely similar condition to that above described*, it is probable that, if ossification 
takes place at all, it is of a very imperfect character. 
Both petro-tympanic bones are unfortunately absent from the skull. ‘The fossa 
at the base of the cranium for. their lodgment is shallow, and the aperture left 
in the cranial wall by their removal large, compared with that in an ordinary Dol- 
phin. It is irregular, circular, and averages 1" in diameter. In the largest skull 
in the British Museum these bones are present, and enter considerably into the forma- 
tion of the cranial wall, the inner and upper surface of the petrosal being seen in 
the interior of the cerebral cavity, on a level with the internal surface of the other 
bonesf. 
One circumstance in which the petro-tympanic bones of Inia differ from those of 
Platanista is their loose connexion with the rest of the cranium; for they are only 
attached by ligament, as in Delphinus, and not locked in their place by a process of 
the mastoid. In general form the tympanic bull resemble those of Delphinus, though 
they are larger than in a member of that genus of corresponding size, and have their 
anterior (Eustachian) extremity rather more prolonged and pointed, though to a far 
less degree than in Platanista. ‘Their antero-posterior length in the adult skull is 
1:65", their greatest breadth 1-1". 
The mandible presents a remarkable miniature resemblance to that of a Cachalot. It 
differs from the mandible of all the true Delphinide by the great length, narrowness, 
and shallowness of the symphysial portion, which includes three-fourths of the tooth- 
bearing part of the rami. The consequence is that the hinder parts of the rami diverge 
much more rapidly from each other than in the true Dolphins. The coronoid process 
is unusually elevated. The lower jaw of Platanista, as is well known, presents all these 
characters, but in a much more exaggerated degree. 
The characteristics of the teeth have been well described by d’Orbigny and Gervais. 
They are distinguished from those of all other Cetaceans by the peculiar and very 
* In the smaller skull in the same Collection nearly the whole of the pterygoids have been destroyed. 
+ After noticing that in certain Delphinoids the aperture left between the hinder edge of the alisphe- 
noid, the exoccipital, basioccipital, and basisphenoid is exceedingly small, so that the tympano-periotic is 
still more shut out from the cranial cavity than in Bulena, Professor Huxley remarks that “in Platanista 
the aperture is large, and the periotie appears in the interior of the cranial cavity in the ordinary way ” (Ele- 
ments of Comparative Anatomy, 1864, p. 276). This is certainly the case in the two small Platanista skulls 
in the Museum of the College of Surgeons, upon which the observation was founded ; but it is worthy of note 
that in a large and apparently aged skull of an individual of the same genus in the British Museum the 
periotic bones are completely shut out of the cerebral cavity by the excessive development of the proper cranial 
bones, and communicate with it only by a narrow passage fully an inch in length. Whether this difference 
depends on age or on species I cannot at present determine ; but it shows that the relative position of these bones 
to the rest of the cranium may yary, even in most closely allied forms, _ 
