520 CETACEA. 



rounded process, concave from behind forwards in its hinder surface which forms 

 the anterior wall of the auditory meatus. In position it is considerably below the 

 upper border of the larger bulla, which arches inwardly over it and into the cavity 

 of which it projects, but leaving a fissure between its upper border and the arched 

 posterior margin of that bulla, the fissure being continuous with the cavities of the 

 larger bulla and tympanic. The cavity of the larger bulla is vertically oval and 

 looks inwards, the greater part of the hinder wall and the floor being formed by 

 tlie smaller bulla ; this latter sends inwards forwards and upwards a curved ridge 

 from its base, a ledge-like floor to the cavity which is much antero-posteriorly 

 compressed and opening freely in the dried skull into the tympanic cavity. It does 

 not occupy the whole of the bulla, for from the floor-ridge of the smaller bulla a 

 fine ridge is prolonged upwards, defining its inner limit, internal to which there 

 is a broad shallow groove running downwards behind its inner extremity where 

 the malleus is attached, viz., transverse to the longitudinal axis of the tympanic and 

 corresponding to the external buUate band. The inner surface of the cavity of 'the 

 bulla in the recent skull is external to the tympanic membrane. This membrane 

 is attached along the upper margin of the basal ridge (floor) of the smaller bulla, 

 and the fine ridge prolonged onwards from it along the inner margin of the cavity, 

 from the upper extremity of which it is reflected backwards to a ridge on the under 

 surface of the rough post-auditory process of the tympanic. Thence it continues 

 to the upper border of the tympanic, between that process and the smaller 

 bulla, and across the base of the latter internally, back to its basal ridge, but 

 above the opening into the cavity of this bulla, which latter is internal to 

 the tympanic, and a short, blind canal with a moderately wide orifice into the 

 tympanic cavity directed backwards. The inner surface of the cavity of the larger 

 bulla consists of a fine network of delicate ridges, the majority of which are 

 transverse, many of the intervening sulci being covered with very small pits. 

 A minute canal occurs in the floor of the cavity, and traversing the external 

 portion of the base of the smaller bulla, opens on the outside of the tympanic in 

 the lower end of the furrow, defining the posterior external border of the bullate 

 band. 



I am indebted to Dr. Doran' for the following account of the ossicles of the ear of 

 Tlatanista : "In the Gangetic dolphin {JPlatanista Gangetica) the head of the malleus 

 is of the same form as in Delphinus, and the upper facet is larger than the lower ; but 

 the process in front is extremely elongated so as to be longer than the head itself, 

 from which it is hardly more distinct than in Monodon, except that a faint groove, 

 generaUy observed in the true dolphins, divides them above and internally. Instead 

 of a rough and only slightly concave surface at the very extremity of the process, 

 as is found in most Cetacea to mark the insertion of the tensor tympani, there is a 

 very deep pit on the external aspect of the projection actually nearer its root than 

 its point; this is highly characteristic. The point itself is sometimes slightly 

 hooked, and from it to close under the articular surface, all along the inner aspect 



' From Doran, Trans. liina. Soc, vol, i (2 ser.), Zoology, Part vii, 1878, not yet printed- 



