PHYLOGENY OF THE PAL^EOG-NATHJE AND NEOGNATHJ2. 255 



the bronchus a second much slighter and less prominent fold, slightly concave 



forwards, running somewhat obliquely backwards and downwards, and supported by 

 the anterior ends of the third and sixth bronchial semirings." 



There are no intrinsic muscles. The stemo-tracheales in Casuarius and in Dromceus 

 expand so as to meet in the middle line in front. 



The tracheo-bronchial region of the trachea in the adult Dromceus has not yet been 

 described. In a young specimen dissected by Forbes there seemed to be no great 

 differences between it and Casuarius. The membrana semilunaris seemed to be better 

 developed. 



The trachea of Dromceus is peculiar, in that its front wall is pierced by a long, 

 vertical, slit-like median aperture, caused by the deficiency of certain of the tracheal 

 rings in this region. The lining membrane of the tube projects from this opening 

 and forms a large dilatable sac, which can be filled with air at will. It is probably 

 concerned with the production of the drumming sound which this bird occasionally 

 makes. It has been described in detail and figured by Murie [63] ; but seems to have 

 been first discovered and described by Peter Fremery [21] in 1819. 



In Apteryx, it will be remembered, the syrinx is but feebly developed, and the intrinsic 

 muscles are degenerate. The intrinsic muscles of Rhea were first correctly described 

 by Forbes [20]. The last four or five, or even six, tracheal rings fuse more or less 

 completely to form a box. The number of rings involved in this fusion can generally 

 be traced by the interannular sutures, which appear never to be completely obliterated. 

 There is a complete and cartilaginous pessulus and a membrana semilunaris. Forbes 

 describes, and I am able to confirm, " on the middle part of the membrana tympani- 

 formis interna a thickened fibro-cartilaginous area." This is triangular in form, its 

 base resting on the pessulus, and its apex terminating near the 4th bronchial semiring. 

 It would be interesting to compare this thickened tissue with certain fibrous pea- 

 shaped pads found in this region in Sula bassana, first described by Garrod, and 

 Catarrhactes chrysocome, described and figured by Watson in the ' Challenger ' Reports. 



The broncho-trachealis (intrinsic syringeal) muscle in my dissections, as in Forbes's, 

 arises from the last ten tracheal rings, and passes downwards to be inserted into the 

 anterior ends of the last four or five bronchial semirings and the membrane between. 

 The sterno-trachealis terminates just pre-axiad of the origin of the intrinsic muscle. 



In the Casuariiclce, as Forbes has pointed out, the " terminal part of the trachea tends 

 to develop into an expanded tympanum " ; the tracheal rings are softer and become 

 imperfect posteriorly, the junction of the trachea and bronchi is less marked, and the 

 arrangement of the sterno-tracheales is peculiar. 



In SirufMo and Apteryx the tendency of the trachea is to become narrower at its 

 termination, and the distinction between this and the bronchi is much sharper than in 

 Casuarius. 



2n 2 



