1910.] ANJ> IJIOI-UGV OK Till-: LARGER CETACEA. 779 



and shapeil like a cowrie shell. It occupies the po.sterior half of 

 the depression, into which it fits fairly closely (text-iig. 71, a, c), 

 its outer inferior surface being level with the projecting edge of 

 the basioccipital, the pterygoid fossa lying innnediately in front. 

 The tympanic bone may be said to hii,ve iin inner and an outer 

 surface, which meet below. The inner surface is fiat, its lower 

 edge lying near, but not touching, that of the projecting process 

 of the basioccipital ; its upper edge is smooth and rolled into the 

 cavity of the bone. The outer side of the tympanic is I'ounded, 

 with the convexity pointing outwards. The upper e(]ge of this 

 outer side forms an irregular extended lip, which gives rise to the 

 anterior and posterior pedicles. On this lip, immediately behind 

 the anterior pedicle and at right angles to it, there is a ridge 

 projecting upwaixls and nearly touching the periotic. The inner 

 edge of this ridge is continuous with the processus longus of the 

 malleus, the malleus thus being fused to the tympiinic bone. 

 This process, which runs j^arallel to the posterior pedicle, together 

 with the latter, foi-m the sides of the auditory meatus. 



The malleus has been described by T)vs. Carte and Macalister*, 

 and is stated by these authors to be fused to the tynipanic bone 

 by the handle or manul)rium ; whereas Dr. Dwight t regards that 

 portion of the malleus which has co-ossified with the bulla as the 

 processus longus or gracilis. The latter would seem to be the 

 correct interj)retation judging from the mode of attnchment of 

 the tympanic membrane. This structure was apparently lost or 

 had decayed in the specimens dissected by the above-mentioned 

 writers, so they could only guess at the relationship of the parts. 

 The manubrium is reduced to a short process, slightly hooked at its 

 distal end (PL LXXIY. fig. 1, b), on the ventral surface of which 

 is attached the ligament of the tympanic membrane. This is the 

 only a.ttachment of the membrane to the malleus. The lim of 

 the tympanic membrane sac is joined to the inner extremity of 

 the external aiiditory canal. The processus longus or gracilis 

 (PI. LXXIV. fig. l,c) is well developed, and is fused to the inner 

 edge of the lip of the tympanic bulla. The incus and sta.pes have 

 been described by previous wi-itersj, and are morphologically 

 similar to those found in other Mammals. 



At the anterior end of the tympanic cavity, in front of the 

 anterior pedicle of the tympanic bone, there is an opening which 

 communicates with the pterygoid fossa. This maybe regarded as 

 the enlarged inner end of the Eustachian tube. The pterygoid 

 fossa, which measures from 6 to 8 inches in length, about 4 inches 

 in width, and 4 inches in depth, is also a portion of the Eustachian 

 tube, for it is lined by the same mucous membrane as the tymjianic 

 cavity and the Eustachian tidie proper. The latter is a relatively 

 narrow canal, about three-quarters of an inch in diameter, which 

 opens out of the floor of the pterygoid fossa and -winds along a 



* CavtP & Miicalister, lor. cit. p. 254. 



t Dwi-ht, Mcin. Boston Soc. Nat. Hist. 1872, vol. ii. p. 226. 



I Carte & Macalistei-, loc. cit. p. 254; Dwight, 1<k-. vit. p. 226. 



