REPORT ON THE ANATOMY OF THE PETRELS. 39 
semi-rings anteriorly, their backward prolongation fusing behind with cartilaginous 
elements developed in connection with the posterior ends of the second semi-rings, and all 
ultimately fusing into the pessular box of the adult. In Puffinus obscwrus there is a 
complete pessular bar, formed by the third bronchial rings. There is no pessular box, 
the bronchial rings being all free from each other. 
Fic. 24. —Syrinx of Pufinus brevicauda, Fic. 25.—The same, from behind. Fic. 26.—The same, from a younger bird, 
from before. in which the bronchial and tracheal 
rings have not yet coalesced. 
In Gstrelata lessoni (figs. 27, 28) the syrinx becomes much more specialised and 
ossified. The fifth bronchial rings are strong and curved, and to these are attached, by 
fan-shaped insertions of tendon, the vocal muscles. The five preceding rings (which 
must therefore be the last tracheal and first four bronchial) are narrow, closely united, 
Fie. 27.—Syrinx of @strelata lessoni, from before. Fic. 28.—The same, from behind. 
and ossified over a rhomboidal space in the middle line anteriorly. Behind there is a 
pessular box formed by these rings, and the four preceding tracheal ones in addition, 
the first two of these having a median patch of ossification. There is a well- 
developed and ossified pessular bar. 
