498 CETACEA. 



presents a close relationship to this form, differs widely from it in these details, as 

 does also Fontoporia. 



As pointed out by Cuvier, the petrotympanic is not merely suspended by means 

 of fibrous tissue, but is locked into position by a process of the mastoid, whereas in 

 Inia, Elower describes these bones as loosely attached to the cranium, and from 

 their absence in the skull of Fontoporia he concludes that their attachment must be 

 much the same as in Inia. In Flatanista, on the other hand, the Eustachian 

 extremity of the tympanic is long and tubular. In Inia this feature is much less pro- 

 nounced, and the bone partakes more of the character that distinguishes it in 

 DelpMnus. 



The resemblance of the lower jaw to that of the Cachalot has been forcibly 

 pointed out by Cuvier, Eschricht 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 Tlatanista 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 Elower ^ and 

 Burmeister. ^ 



Comparing the vertebral columns of Flatanista, Inia and Fontoporia, great 

 differences, besides the fewer vertebrae in the two latter as compared with the 

 former, are found to obtain. The most marked feature of Flatanista is the, so 

 to speak, enormously developed oblique processes by means of which the individual 

 vertebrae from the fifth dorsal backwards for a long way are interlocked with strongly 

 f orwardly 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 JPlatanista is absent, 

 but while in Fontoporia 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 Inia 

 is the unusual length of its neck, which is nearly as weU developed as in many quad- 

 rupeds. The cervical vertebrae are marked by considerable transverse extension 

 and by depression, and in these characters they differ from the vertebrae of Inia, 

 while, however, as in Inia and Fontoporia, they are all free, but the neck of this 

 last mentioned Cetacean is considerably shorter than in Inia. 



In the atlas, as in Fontoporia, ^ the spinous process is feebly marked, and there is 

 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 



^ Loc. cit. 



- Annales de Mus. Pub. de Buenos Aires, 1869, PI. xxvii, figs. 2a and 3a. 



' Burmeister: loc. cit. 



