ART. 19 A MIOCENE PORPOISE FROM MARYLAND KELLOGG 25 



side, and extends downward upon the external face of the pars coch- 

 leaHs. Along the internal margin of this fossa a thin-edged crest is 

 developed on the ventroexternal angle of the jyars coehlearis which 

 extends backward to the posterior margin. In position and shape the 

 stapedial fossa of this fossil periotic is essentially the same as on the 

 periotic of Prodelphinus tnalayanus. The depth of the posterior 

 face of this fossil periotic (7.1 mm.), as measured from the stapedial 

 fossa to the fossa for the cerebral orifice of the Aquaeductus vesti- 

 btdi, however, is almost twice that of the living porpoise (8.9 m.). 



Between the rounded tuberosit}'^ or swelling on the basal portion 

 of the anterior process and the anterior margin of the articular facet 

 on the posterior process the ventral surface of the external denser 

 portion of the periotic is deeply excavated. The raised external 

 margin of the fossa incudis shuts off this excavation from the epi- 

 iympanic recess, paralleling DelpMnodon dlvidum, but differing 

 from both Prodelphinus vKjlayanus and Prodelplmvus longirostris 

 in this respect, for in these living porpoises the fossa incudis is 

 shorter and the excavation or groove is continuous with the surface 

 between the fossa for the head of the malleus and the epitympanic 

 orifice of the Aquaeductus Fallopii. The narrow fossa incudis. 

 which receives the ams hreve of the incus, extends the full length 

 of the thin ledge which projects inward below the canal for the 

 facial nerve. The anterior half of the fossa\ incudis is shallowly 

 concave and almost horizontal, but posteriorly it terminates in a 

 small deep pit or ovoidal concavity on the anterointernal angle of 

 the posterior process. The anterior border of the projecting ledge 

 for this fossa is free and above it but external to the epitympanic 

 orifice of the Aquaeductus Fallopii is a small depression. 



The anterior process is robust, almost triangular in cross section, 

 and is obliquely truncated anteriorly. This process bends inward 

 and is slightly twisted. The main articular surface for the ac- 

 cessory ossicle or uncinate process of the tympanic is a broad ovoidal 

 area which occupies the median portion of the ventral surface of 

 the anterior process. This articular surface curves from end to end 

 and slopes toward the pars cochleans. On the adjoining ventral 

 surface of the pars cochlearls there is a circular concavity which is 

 supplementary to the main articular surface on the anterior process. 

 When the accessory ossicle is in position the groove between the 

 anterior process and the pars cochlearis is completely closed. On 

 the periotic of Prodelphinus^ however, a narrow groove is left open 

 between the accessory ossicle and the pajrs cochlearis. The concave 

 fossa for the head of the malleus occupies the internal face of the 

 tuberosity or swelling on the basal portion of the anterior process, 

 ooni — 27 4 



