786 MR. W. A. FORBES ON THE [Juiie 21, 



The exact number of tracheal rings which are fused to form this box 

 varies in different specimens from four to six ; in some cases it is, 

 apparently, formed by four rings on one side and five on another. 

 The lowest tracheal ring is strongly concave downwards, but in front 

 in the middle line is transversely truncated. Posteriorly the tympanic 

 box is deeply and widely notched. There is a distinct, narrow, 

 cartilaginous pessulus, which runs from behind forwards, connecting 

 the anterior and posterior walls of this box, and interrupting, in 

 the mid line, the continuity of the memhrana tympanifornis, which 

 completes the bronchial walls internally. 



The first bronchial semiring is nearly straight, and of cylindrical 

 form. It is closely connected at its extremities with the last tracheal 

 ring ; but between these points is a pretty wide, lunate, interannular 

 interval. The anterior ends of these first semirings are inturned con- 

 siderably, but do not meet each other, or the pessulus ; posteriorly 

 they do not extend inwards, by some way, as far as the preceding or 

 succeeding rings. The second bronchial semiring is similar in form, 

 but dilated slightly behind ; it is closely approximated to the pre- 

 ceding ring in the greater part of its extent, but is anteriorly strongly 

 curved downwards (in a somewhat sinuous way), so that here a con- 

 siderable space is left between the two semirings in question. The 

 third and fourth rings are considerably dilated, and produced in- 

 wards, posteriorly ; anteriorly, in the particular specimen figured, 

 they are fused into a comparatively narrow ring. The fifth, sixth, and 

 succeeding semirings are quite simple incomplete hoops of cartilage, 

 which become smaller and less complete internally as they approach 

 the lungs. 



Between the pessulus in the middle line and the tracheal box and 

 first four bronchial semirings, is spread a membrane, completing the 

 tracheal and bronchial walls at the bifurcation of the tube. This 

 memhrana tympaniformis is thinnest posteriorly; but in the middle 

 part of its extent, over a triangular area — the base of the triangle 

 resting on the pessulus, whilst the apex is at the posterior end of the 

 fourth bronchial semiring — it is thickened and of a fibro-cartilaginous 

 consistency. 



From the sides of about the last ten tracheal rings a single thin 

 but broad band of muscle arises, the fibres of which, running down- 

 wards and forwards, are inserted into the anterior halves of the first 

 five bronchial semirings and the membranes between them, extending, 

 in some specimens, almost to the anterior tips of the first two semi- 

 rings. This muscle is therefore in all respects a true intrinsic 

 syringeal one. The lateral tracheal muscle stops anteriorly to the 

 origin of the intrinsic one. 



Internally, there is a well-marked, sharp-bordered memhrana 

 semilunaris, resting on the pessulus, developed between the bronchi. 

 In the space corresponding to the interval between the last tracheal 

 and first bronchial rings, an accumulation of the fibrous and elastic 

 tissues of the living membrane of the bronchus gives rise to a well- 

 developed vocal cord. 



Rhea macrorhyncha closely resembles, in all respects, B. americana 



