408 STRUCTURE AND CLASSIFICATION OF BIRDS 



is flattened 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. 



Phakicrocorax has a complete 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. brasiUetislx. 



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 

 GAEHOD'S figure. 1 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, 



1 P. Z. S. 1870, pi. xxxviii. fig. 4. 



