MIOCENE PORPOISES — ^KELLOGG 129 



centrum is shallowly concave, and the posterior surface convex. 

 The centrum is pierced dorsoventrally by a pair of canals, the dorsal 

 orifices of which are 9 mm. apart. 



RIBS 



Although all of the ribs (pi. 10) associated with these skeletal 

 remains are more or less incomplete, there is evidence that this fossil 

 porpoise possessed at least 10 pairs of ribs and possibly 13, of which 

 the first pair are short and robust. When these ribs are arranged in 

 what appears to be their normal position, the abrupt downward 

 bending of the shaft behind the neck is seen to diminish from the 

 anterior to the posterior end of the series. 



For the first rib (pi. 10, fig. 1) on the left side only a section of the 

 curved portion of the shaft behind the tuberculum was found. The 

 neck is flattened, and relatively deep behind the tuberculum of which 

 only the hinder border of this articular surface is preserved. The 

 downward bending of the shaft of the rib behind the tuberculum 

 apparently was not as abrupt as on the corresponding rib of Inia. 



The right and left second ribs (pi. 10, figs. 2, 7) both lack their 

 distal extremities, but are otherwise somewhat similar to the ribs of 

 Inia. The necks of these ribs are short, the capitular portion being 

 slightly bent upward. The tuberculum is elongated, with greatest 

 width posteriorly (11.5 mm.) and attenuated anteriorly. Between 

 the tuberculum and the angle the posterosuperior margin overhangs 

 interruptedly the posterior surface of the shaft. The downward 

 bending of the flattened shaft is more pronounced than on the suc- 

 ceeding ribs. 



Only the proximal end of the third rib (pi. 10, fig. 10) on the right 

 side was found. The neck is narrower and slightly longer than that 

 of the second rib. The convex capitulum is subovoidal in outline. 

 The tuberculum is elongated, subpyriform in outline, and concave 

 from end to end. 



The necks of the right and left fourth ribs (pi. 10, figs. 3, 8) are 

 slender, less flattened, and more elongated than that of the third rib. 

 The capitulum is broken off on the left rib and is incomplete ventrally 

 on the right rib, but was obviously smaller than that of the third rib. 

 The slightly convex tuberculum is extended anteriorly on the neck. 

 Between the tuberculum and the angle the shaft of this rib is antero- 

 posteriorly flattened, but becomes more nearly ovoidal in cross sec- 

 tion toward the broken distal end. The angle overhangs the posterior 

 face of the shaft. 



The fifth rib (pi. 10, fig. 4) on the left side is characterized by a 

 slender neck, an expanded capitulum, and an elongated tuberculum 



