408 STRUCTURE AND CLASSIFICATION OF BIRDS 



is fjattened from before backwards, and the first two bronchial 

 semi-rings are very prominent. To the first of them, and 

 apparently also to the ring in front, are attached the two 

 muscles into which the intrinsic muscle divides. There is 

 a membranous gap separating the last of the specialised 

 bronchial semi-rings from the first of those which follow, 

 whose border, moreover, is concave upwards. The last few 

 tracheal rings are ossified and firmly fused. 



PhalacroGorax has a coniplete bronchidesmus and a single 

 pair of intrinsic muscles. The first three bronchial semi- 

 rings are very prominent and arched, and to the third of 

 these the intrinsic muscles are attached. There is a mem- 

 branous gap between the last of these and the first of the 

 remaining series of bronchial semi-rings, which forms, at any 

 rate in P. carbo, quite a pocket. The fourth bronchial semi- 

 ring is curved in the same direction as that which precedes 

 it ; both, in fact, are convex. The curvature, which is slightly 

 more marked in P brasiliensis than in either P. carbo or 

 P- varius, suggests very much the syrinx of certain auks 

 (cf. p. 363). In P. varius the intrinsic muscles are attached 

 to the second bronchial semi-ring, as also in P. brasiliensis. 



The syrinx of Plotus does not differ greatly, but it has an 

 incomplete bronchidesmus. There are two bronchial semi- 

 rings, which are specially increased in length and depth ; 

 they are the second and third, and are relatively stouter than 

 those of Phalacrocorax ; to the first of them the intrinsic 

 muscles are attached. 



The syrinx of Sula is a good deal different from that of 

 other steganopods. 



There is no ossification, except in the pessulus. A square 

 projection is formed by a fusion between last tracheal rings ; 

 this is continuous with the pessulus and is well shown in 

 Gaeeod's figure.' The bronchial semi-rings are at first 

 feeble with wide interval. Between the third and fourth of 

 them there is, covering the insertion of the intrinsic muscle, 

 a protuberant pad of elastic tissue about the size and shape 

 of a pea. 



• P. Z. S. 1876, pi. xxxviii. fig. 4. 



