MIOCENE PORPOISES — KELLOGG 127 



postzygapophysial facets are quite similar to those on the latter. 

 The facet for the capitulum of the seventh rib is large and situated on 

 the centrum at the upper posteroexternal angle. The anterior surface 

 of the centrum is narrower than the posterior surface. 



Eighth dorsal: The prezygapophyses (pi, 9, figs. 3, 4) are separated 

 by a shorter interval, the neural canal is noticeably smaller, and the 

 backwardly projecting dorsal portion of the neural arch is very narrow 

 and more noticeably attenuated than on the sixth dorsal. The anterior 

 and posterior surfaces of the centrum are approximately the same 

 width. 



The anteroposterior diameter of the basal portion of the neural 

 spine exceeds that of the corresponding dorsal of Eurhinodelphis, but is 

 approximately equal to that of Inia. The short diapophyses differ 

 very slightly in position from those of the sixth dorsal, although the 

 facets for articulation with the tubercula of the eighth ribs are almost 

 triangular in outline. The concave prezygapophysial facets are small, 

 measuring about 16 mm. anteroposteriorly, and slope very abruptly 

 from outer to inner margins. The somewhat curved and slender 

 prezygapophyses project about 17 mm. beyond the anterior margins 

 of the ventrally situated articular facets. The postzygapophysial 

 facets are similar in outline to those of the sixth dorsal, but are shorter. 

 The centrum lacks any indication of a facet for the capitulum of the 

 ninth rib. On the centrum of the corresponding dorsal of Inia, how- 

 ever, the large facet for the capitulum of the eighth rib is located on the 

 upper anteroexternal angle of the centrum and is continuous with the 

 facet for the tuberculum. 



Tenth dorsal: The eighth and ninth dorsal vertebrae of Inia 

 have the single large facet for the combined tuberculum and capitulum 

 of the corresponding rib located for the most part on the lateral surface 

 of the pedicle of the neural arch. On the tenth dorsal of Inia, however, 

 this facet is located largely below the level of the dorsal surface of the 

 centrum. The transverse processes of these vertebrae of Inia are 

 very short. This fossil vertebra (pi. 9, figs. 5, 6) thus differs from 

 the tenth dorsal of Inia in having the single large concave articular 

 facet for the combined tuberculum and capitulum of the corresponding 

 rib located laterally on the rather robust transverse process, which 

 projects outward from the pedicle of the neural arch at a lower level 

 than on the eighth dorsal. The distance from the inner surface of the 

 pedicle of the neural arch to extremity of the transverse process is 

 27 mm. The anteroposterior diameter of the neural spine at the 

 base is greater, the neural canal is narrower, and the prezygapo- 

 physial facets are much more closely approximated than on the 

 eighth dorsal. The concave prezygapophysial facets slope abruptly 



