36 



THE VOYAGE OF H.M.S. CHALLENGER. 



between the similarly ossified and fused first three bronchial rings. The fourth pair of 

 rings is also ossified, but free from those that precede it, at least anteriorly. Posteriorly 

 the first four bronchial and last four tracheal rings are firmly co-ossified into a large 

 pessular box, whilst ossification in the median line (both before and behind) of a number 

 of the cartilaginous tracheal rings above this forms the line of attachment for the median 

 septum which divides the tracheal tube for an extent of about 1"25 inch. 



In Fulmarns glacialis the syrinx is very similar ; the four bronchial rings are 

 anteriorly ossified, as are many of the tracheal rings in their median area. The tracheal 

 septum is still more developed than in Thalassceca, extending for about the lower two- 

 fifths of the trachea — a distance of nearly 1\ inches. 



The extraordinary syrinx of Ossifraga is a still further modification of this type 

 (figs. 17, 18, 19). A number of the last tracheal rings (nine on one side, ten on the other, 



Fig. 15. — Syrinx of Thalassceca glacialoidcs, from before. 



Fig. 16. — The same, from behind. 



in the specimen — a young one — figured) become completely ossified, as are the first four 

 or five bronchial rings, of which only the first two are complete. The inferior part of the 

 trachea is divided inferiorly for a short way into two quite complete and separate tubes ; 

 the posterior ends of the lowest tracheal rings being so much incurved that each actually, 

 inferiorly, forms two complete rings, those of opposite sides being quite separate, whilst 

 above, by the gradual diminution of their opposed interior halves, they become, when 

 viewed from outside, simple rings of the normal tracheal type. On section, however, it is 

 seen that then ends are still incurved to form a tracheal septum like that of Fulmartcs 

 and T/talassceca. This completely divides the trachea into two tubes for a space of about 

 3^ inches, terminating above by a free semi-lunar border, concave upwards (fig. 19, c, d). 

 In fig. 19 three sections are given of the inferior portion of the trachea made along the 

 lines a, b, c in fig. 17, to show how the two tracheal tubes, separate below, gradually 

 unite above. 



