EASTMAN : SHARKS' TEETH AND CETACEAN BONES. 91 



this aspect is identifiable with the petrous bone of other mammals, and 

 has the usual pointed anterior extremity (9). Immediately behind this 

 is seen a tumid shelf-like projection which overhangs and partly conceals 

 the anterior process of the bulla, together with its " accessory ossicle," 

 being in fact slightly fused with the latter underneath. This project- 

 ing portion of the front margin is called the processus anterior petrosi 

 (10) ; and underneath it passes in a longitudinal direction the tensor 

 muscle of the tympanic. The hinder portion of the petrous body forms 

 the posterior process (11), by which, as we have seen, the periotic and 

 tympanic are principally held together. A portion at least of this 

 structure is evidently equivalent to the processus mastoideus petrosi of 

 other Mammals. About midway between the two processes referred 

 to, a sinus occurs in the supero-external margin of the periotic which 



/6-_ 

 19 



3 



Fig. B. — Left tympano-periotic of the existing Dtlphinapterus leucas Pallas. Superior 

 face, x I- 



affords ingress by means of an obliquely descending semicircular conduit 

 (12) into the tympanic cavity. This passageway is named by Beaure- 

 gard ductus petro-tympanicus ; by Boenninghaus, who regards it as 

 peculiar to Cetacea, it is called hiatus epitympanicus. 



Cerebral or inner face (Fig. C). — Viewed from this position, the nar- 

 row inner lip (16) of the bulla appears uppermost in the figure (in 

 reality it is lowermost), and immediately below this, yet separated from 

 it by the continuous tympano-periotic fissure, is the rounded portion of 

 the periotic which contains the cochlea. The windings of the scala may 

 be readily followed, the outer wall being formed by the promontory, and 

 the inner spirals appearing within the large funnel-shaped pit known as 

 the porus acusticus internus (15), which gives passage for the acusti co-facial 

 nerve. In the present form, as indeed in most Delphinoids, the aperture 



