418 ON THE PETRELS COLLECTED DURING 



aperture in each centrum. The transverse processes, too, are in these 

 latter birds very much hollowed out for air-cavitiea. 



The ribs in the Oceanitidae are peculiarly broad, and flattened out 

 dorsally, to an extent not seen in any Procellarian. 



In Ptlecandides the ribs are very long, and oblique in position, the 

 more posterior ones most so, with the angles formed by their vertebral 

 and sternal moieties very acute. Thu& the whole trunk almost becomes 

 completely surrounded by a bony box, in a way well calculated to resist 

 the pressure of the water when these birds dive. The same modification 

 may be seen well-developed in the diving Alcidae (Uria, Alca, &c.). 



The uncinate processes are well-developed and nearly straight. They 

 are firmly anchylosed to the ribs. 



As may be seen from the table, the number of ribs and unciuate pro- 

 cesses varies slightly, and the same is true for the sacral and caudal 

 Zool. Chall. vertebrae. The latter have well-developed transverse processes, and 

 ot'xJ 47 between their centres inferiorly small chevron bones, smallest anteriorly, 

 larger and double posteriorly, are developed. The last of these may, 

 apparently, anchylose with the body of the corresponding vertebra. The 

 pygostyle is long and compressed. The diving Pelecandides has, it will be 

 noticed, a greater number of vertebrae (9) in its tail than the other forms. 



Pectoral Arch. The sternum (Pis. XXIII. & XXIV,) is usuallyfrather 

 broad and short, much longer in Pelecandides than in any other genus, 

 with a well-developed keel, and a short, but distinct manubriuin obsolete 

 in Pelecandides. The costal processes are triangular in shape, directed 

 outwards, or in the Oceanitidae and Pelecandides, forwards and outwards. 

 The anterior margin of the keel is more or less excavated, with its lower 

 angle produced forwards, most so in Puffinus anglorum. In Pelecandides 

 (PL XXIII. figs. 3, 4) this part articulates with] the clavicular symphysis, 

 instead of being merely connected to it by ligament, as in the other 

 forms. The coracoidal grooves are oblique backwards, and present two 

 distinct articular areas for the articulation of the coracoid bone. The 

 sides of the sternum usually converge towards the lower end of the 

 costal process, and then diverge again to their posterior extremities. As 

 Ibid. p. 48. may be seen from the figures of Plates XXIII. & XXIV., the posterior 

 end of the sternum varies a great deal in its outline in different members 

 of the group. 



In most of the larger forms of Procellariidae, the visceral aspect 

 of the sternum presents, towards its anterior extremity, more or fewer 

 pneumatic apertures, which are best developed in the Albatrosses, where 

 the whole bons is much permeated with air. In Fulmarus y and all the 

 smaller forms of Procellariidae, as well as in the Oceanitidaa, the sternum 

 has no pneumatic apertures at all, and does not contain air. 



In the genera Cynwchorea (PI. XXIV. fig. 7), Procellaria, and Halocyp- 

 taia, as also in Fregetta (fig. 9) and Garrodia, the posterior margin of the 



