PLA TANISTIDsE 



259 



iii.. 1 Teeth variable, from 26 to 33 on either side of each jaw; 

 those at the posterior part with a distinct tubercle at the inner side 

 of the base of the crown. Vertebrae : C 7, D 13, L 3, C"18 ; total 

 41. Transverse processes of lumbar vertebrae very broad. Sternum 

 short and broad, and consisting of a single segment only. Dorsal 

 fin a mere ridge. The long cylindrical rostrum externally furnished 

 with scattered, stout, and crisp hairs. One species only is known, 

 /. geoffroyensis, about 7 feet in length, inhabiting the upper Amazon 

 and its tributary streams. 



Pontoporia. 2 Teeth 50 to 60 on either side of each jaw, with a 

 cingulum at the base of the crown. Jaws very long and slender. 

 Vertebrae: C 7, D 10, L 5, C 19 ; total 41. Transverse processes 

 of the lumbar vertebrae extremely broad. Sternum elongated, 

 composed of two segments, with four sternal ribs attached. Dorsal 

 fin rather small, triangular, pointed. External respiratory aperture 



Fio. 89. Pontoporia blainvillei. (From Bunneister.) 



transverse, crescentic. This genus connects the last two forms with 

 the true Delphinidce. The only species, P. blainvillei, is one of the 

 smallest members of the whole order, not exceeding 5 feet in length. 

 It has only been met with at the mouth of the Rio de la Plata, near 

 Buenos Ayres, and there is at present no evidence that it ascends 

 into the fresh waters of the river. 



Fossil forms. Remains of a Cetacean from the Pleistocene of 

 South America were referred by Bravard to Pontoporui, but they 

 have been regarded by other writers as indicating a distinct genus, 

 for which the names Palceopontoporia and Poiitistes have been pro- 

 posed. The Upper Tertiary European genera Champsodelphis and 

 Schizodelphis are generally referred to the present family. The 

 former has wide transverse processes to the lumbar vertebras, as in 

 Inia, while the teeth also resemble those of that genus. In Schizo- 

 delphis the form of the rostrum presents a great resemblance to that 

 of the Delphinoid genus Stem, but the symphysis of the mandible 

 is relatively longer. A number of fossil Cetaceans from the 

 Miocene of the United States, such as Priscodelphinus, Lophocetus, 

 Ixacanthus, Rhabdosteus, etc., are referred by Professor E. D. Cope to 



1 D'Orbigny, Nouv. Ann. Mus. Paris, vol. iii. p. 31 (1834). 

 2 Gray, Zoology of Erebus and Terror, p. 46 (1846). 



