MIOCENE PORPOISES — KELLOGG 117 



external denser portion of the periotic is deeply excavated, but is 

 almost shut off internally from the epitympanic recess by a thin 

 osseous crest which constitutes the external boundary of the small fossa 

 incudis. The fossa incudis, which receives the crus breve of the incus, 

 is shallowly concave anteriorly and terminates in a small deep pit on 

 the anterointernal angle of the posterior process. 



The anterior process is compressed from side to side, almost tri- 

 angular in cross section and is bent inward, terminating in a ventral, 

 upwardly curved point. The main articular surface on the anterior 

 process for the accessory ossicle or uncinate process of the tympanic 

 bulla is elongated and concavely curved in an anteroposterior direc- 

 tion. This articular area occupies more than half of the ventral surface 

 of the anterior process. A fingerlike osseous projection, which arises 

 between the pars cochlearis and the narrow deep groove which marks 

 the internal limit of the facet for the above-mentioned accessory 

 ossicle, is no doubt fortuitous. 



The cerebral face of the pars cochlearis (pi. 5, fig. 4) is broken off, 

 exposing the scala vestibuli and the scala tympani. Consequently, 

 the conformation of the internal acoustic meatus cannot be determined. 

 A rather broad channel for the facial nerve leads to the orifice of the 

 aquaeductus Fallopii at the antero external angle of the internal 

 acoustic meatus. The flattened foramen singulare is situated behind 

 the low ridge which separates the spiral tract from the orifice of the 

 aquaeductus Fallopii. 



The tractus spiralis foraminosus forms two circular depressions, the 

 hinder one being much deeper than the anterior one. The cerebral 

 orifice of the aquaeductus cochleae is also broken oft', but obviously 

 was quite small, judging from the diameter of the aqueduct. Outside 

 of the internal acoustic meatus and near its posteroexternal angle is the 

 slitlike orifice of the aquaeductus vestibuli that opens into a rather 

 deep triangular fossa. 



External to the internal acoustic meatus, the cerebral face of the 

 outer denser portion of the periotic is noticeably flattened and forms 

 an obtuse angle with the shallowly concave external face. On the 

 posterior face of this denser portion and above the hinder end of the 

 stapedial fossa is a rather large depression, subtrapezoidal in outline. 



The posterior process of the right periotic (USNM 20659) referred 

 to this species is essentially complete and the structures on the 

 cerebral face are undamaged. The articular facet (pi. 5, fig. 2) on 

 the ventral face of the somewhat elongated and distally truncated 

 posterior process of this right periotic is shallowly concave on the basal 

 portion and slopes from the distal extremity to the anterior margin. 

 A few shallow grooves traverse this facet in an oblique direction. 



338378—55- 



