94 ME. "W. H. PLOWEE 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 ii-regular, 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 bullse resemble those of Beljjhinus, 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 I'l". 



The mandible presents a remarkable miniature resemblance to that of a Cachalot. It 

 differs from the mandible of all the true Beljjhinidce 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 CoUection nearly the whole of the pterygoids have been destroyed. 



t After noticing that in certain Delphinoids the aperture left between the hinder edge of the alisphe- 

 noid, the exoceipital, basioccipital, and basisphenoid is exceedingly small, so that the tympano-periotic is 

 stUl more shut out from the cranial cavity than in Balcena, Professor Huxley remarks that " in Platanista 

 the aperture is large, and the periotic appears in the interior of the cranial canity 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 cfnly by a narrow passage fuUy an inch in length. '\Miether this diiference 

 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 vary, even in most closely allied forms. 



