PLATANISTA. 499 



more strongly marked in Inia. A distinct odontoid occurs in Tlatanista, 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 Tlatanista 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 vertebrse of Fontoporia} 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 lontoporia in the same vertebra 

 by the union of the upper and lower transverse processes as described by Elower 

 and Burmeister. Flatanista 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 deve- 

 loped in Inia and Fontoporia, but strongly in Flatanista. 



In Flatanista, 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 suc- 

 ceeding vertebrge 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. On the eighth the head and tubercle have practically coalesced, and it 

 is this vertebra 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. 



It will be seen from this description that the arrangement of the ribs is much 

 the same as in Inia, and as in Fontoporia 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 rib 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. Prom the eighth vertebra backwards each rib in Flatanista is only 

 attached to its own vertebra, whereas in Inia this takes place in the sixth, the 

 coalescence of the articular surfaces for the head and tubercle in Inia, as in Flata- 

 nista, occurring on the eighth vertebra. 



The difference between Syperoodon and Fhyseter and Inia hes 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 



^ Burmeister, PL xxvi, figs. 6-8. 



