Airr. 14. MARYLAND FOSSIL PORPOISE KELLOGG. 29 



These dorsals differ noticeably from all the dorsal vertebrae de- 

 scribed by Cope and Leidy, not only in size, but also in structure. 

 They also differ from many recent delphinoids in that the neural 

 spines are nearly vertical instead of being strongly inclined back- 

 ward or forward {Inia). In the first three dorsals the antero- 

 posterior diameter of the spine at the tip is somewhat less than at 

 the base, this being especially noticeable in the first dorsal. The 

 neural spines of the remaining dorsals, however, are of approxi- 

 mately the same depth throughout. In this respect they resemble 

 the dorsals of Plataiiista more closely than those of Inia or any 

 other recent species available for comparison. 



The anterior dorsal vertebra of this fossil porpoise presents the 

 majority of those features which characterize the first dorsals of 

 Inia geo-ffrensis and Platanista gangetica. This vertebra agrees with 

 that of Inia in the general appearance of the transverse process and 

 the position of the articular facet for the tuberculum of the first rib, 

 but differs in the great depth of the centrum and the relative width 

 and height of the neural spine. 



The anterior dorsal is peculiar as regards the dimensions of the 

 centrum which is almost as long as broad. The epiphyses are rela- 

 tively thin. There is a distinct oval facet for the accommodation of 

 the head of the second rib on the postero-superior lateral margin of 

 the centrum. The neural arch is low, broad, and thin, with a lateral 

 transverse process on each side which bears an articular facet for 

 the accommodation of the tuberculum of the first rib. The articular 

 facet on the transverse process for tubercle of rib is horizontal, that 

 is it is situated on the inferior face of this process. If any reliance 

 can be placed on the position of this articular facet, then this is actu- 

 ally the first vertebra of the dorsal series. 



Compared with the same vertebra in a young Platanista skeleton, 

 the principal differences are as follows: The centrum is longer, but 

 not so compressed dorso-ventrally ; the neural canal is much larger ; 

 and the posterior margin of the neural spine is more arcuate. 



In this fossil series of dorsals, the centra increase perceptibly in 

 length from the firet to the last. The prezygapophysial facets of the 

 first six dorsals are nearly horizontal in position while those of the 

 last four (pi. 13, fig. 1) are oblique. In the first six dorsals the 

 articular surface for the accommodation of the postzygapophysis is 

 circular; in the others this surface is nearly oval. These facets for 

 the postzygapophyses (pi. 13, fig. 2) are situated on the superior face 

 of the upturned margin of the laminae in front of and below the 

 point where the metapophyses arise. As we go backward along the 

 series they gradually shift from a horizontal to a lateral position. 



5596— 24— rroc.N.M.vol.63 17 



