Mtr. 1<> A MIOCENE PORPOISE FROM MARYLAND KELLOGG 31 



The stapes (fig. 18) is of the same form as in Prodelphinus. The 

 intercrural aperture is small and connects the relatively large con- 

 i;avities on the opposite sides. A slight side to side movement is 

 permitted when the stapes is in position, but the footplate is closel}^ 

 fitted to the free margin of the fenestra ovalis. The stapes bears on 

 its footplate (fig. 19) a distinct umbo or large oval concavity on the 

 vestibular face. There is a well-defined scar (fig, 18) on the postero- 

 internal angle below the head which seems to mark the attachment of 

 the stapedius muscle, and this portion of the stapes projects slightly. 

 The facet (fig. 20) which marks the point of contact with the cor- 

 responding facet on the head of the crus longum of the incus is small 

 and is placed obliquely on the head of the stapes. 



MANDIBLES 



Minor differences between the mandibles of this fossil porpoist 

 and those of Sotalia tucuxi show that too much weight should not be 

 assigned to the general shape for purposes of identification. The 

 mandibles (pi. 4) of this fossil porpoise resemble those of Sotalia 

 tucuxi so closely that with the exception of certain points hereinafter 

 mentioned a description of one might apply equally well to the other. 

 These mandibles differ from those of Sotalia tucuxi in that the 

 symphj^sis is longer and from those of Delphinodon dividum in that 

 the posterior margin of the coronoid process above the condyle is 

 more nearly vertical and the number of alveoli is greater. The right 

 mandible has been freed from the matrix, but the left is partially 

 embedded. The mandible is rather long as compared to the skull 

 and the ramus is slightly deeper than that of Sotalia tucuxi. The 

 ramus is slenderest at the posterior end of the symphysis and the 

 tooth row occupies about 65 per cent of its total length. The 

 symphysis is longer than in Sotalia tucuxi., being almost one-third 

 of the length of the mandible, while in the latter it is less than one- 

 sixth. The rami are firmly ankylosed throughout the symphysis and 

 curve upward to the extremity. On the outer face of the mandible 

 and in position corresponding to the middle of the tooth row are 

 five foramina, from each of which a short canal leads forward. The 

 proximal foramen is placed nearest to the superior margin and the 

 distal foramen nearest to the inferior margin; the first-mentioned 

 foramen opens at the level of the fifteenth tooth, counting forward 

 from the last, and the last-mentioned foramen opens at the level of 

 the posterior end of the symphysis. There is a stitcMike or inter- 

 rupted stria which commences on the inferior margin in front of the 

 angle and gradually rises on the outer face as it passes forward until 



