PLATANISTA. 519 



outer half of the hinder margin, and which projects backwards beyond the inner 

 half. 



The superior border of the bone overlooking the cavity is much arched, rising 

 from either extremity and culminating in a transverse buUate eminence, to the base 

 of which posteriorly there is another and smaller buUate process separated by a deep 

 rounded notch, the external auditory meatus, from a low pillar bearing the expanded 

 rough surface to which the tympanic is attached to the post-auditory process. At 

 the inner end of the large transverse bulla, the malleus is suspended by its neck 

 from a firm anchylosis above the processus gracilis. Besides these attachments, a 

 ridge runs outwards and forwards from a pit on the neck of the malleus above the 

 processus gracilis, and seems to be the equivalent of the laxator tympani. Before 

 the ridge and processus gracihs is a deep notch, the Glasserian fissure, anterior to 

 which is a large thick downward and forwardly projecting process like an enormous 

 malleus. Its outer surface forms an upwardly projecting portion of this margin of 

 the tympanic, and is flattened externally with an irregular concavity at its base ; its 

 lower inner border nearly resting on the inner margin of the cavity of the tympanic ; 

 and as the tip of the malleus is close behind it, they define a vertical fissure, a con- 

 tinuation of the Glasserian fissure. Its internal surface is marked by two broad 

 furrows and a central ridge which fit into the external surface of the anterior division 

 of the periotic, the posterior margin of the external surface defining the anterior 

 wall of the Glasserian fissure. In separating the tympanic, this process is usually 

 left attached to the periotic. The two bones in adult skulls are slightly connected 

 at the anterior margin of the external surface of the process, but so very minutely 

 that I could only detect their presence by the aid of a lens. In young skulls there is 

 no trace of anchylosis^ but this process is so thoroughly wedged into the periotic, the 

 mastoid so rooted like a tooth in the post-auditory process, and the vaginal process of 

 the tympanic so locked in between the mastoid, exoccipital, and squamosal, that the 

 bones never separate from the skull except by fracture. 



The large and small bullae require to be described in detail, as they are import- 

 ant structures in the economy of the ear. The former is compressed antero- 

 posteriorly, and occupies the highest point of the upper border of the tympanic, 

 being placed transversely to its longitudinal axis. It is closed anteriorly, but there 

 is a small semi-circular opening into its cavity posteriorly, its hinder waU being 

 incompletely closed ; in the recent skull this opening is external to the tympanum ; 

 it is placed above the level of the posterior and smaller bulla, the cavity of which 

 is internal to the tympanum. The inner extremity of the larger bulla is curved 

 downwards and inwards, and has the malleus attached by its neck to its extremity. 

 The external surface is curved downwards, forwards, and then backwards on the 

 outer side of the tympanic as a prominent buUate band, its anterior border being 

 the larger ; but both it and its posterior border are defined by a deep furrow. 

 Above, where it bends backwards, there is a slight concavity, but the remainder is 

 convex. The posterior furrow is prolonged into the hinder opening before the 

 smaller bulla, which is also an antero-posteriorly compressed transverse smooth 



