488 PKOCELLABIID^: PRION 



with hexagonal scutes ; claw of the hind toe minute ; size small ; 

 plumage slaty- blue. 



This genus, containing the Blue Petrels generally termed 

 " Whale Birds " by Sailors, ranges over the Southern Ocean. Five 

 species, all of which have been reported from the Cape seas, are 

 usually recognised. 



a b c 



FIG. 151. Bills of (a) Prion desolatus, (b) Prion banksi, (c) Prion vittatus, 

 rom above, x T 



Key of the Species. 



A. Outer tail-feathers white, the middle ones tipped 



with white ..................... ........................... P. ccrruleus, p. 488. 



B. Outer tail-feathers grey, central ones tipped with 



black. 



a. Bill very wide, 0*65 to 0'80 at base, edges of 



maxillae convex ........................................ P. vittatus, p. 490. 



b. Bill wide, 0*50 to O55, edges of maxilla? convex P. banksi, p. 489. 



c. Bill narrow, about O40 at base, edges of maxillee 



almost straight ....................................... P. desolatus, p. 491. 



d. Bill narrow, smaller and more compressed, and 



head paler than in the other species ............ P. brevirostris, p. 492. 



797. Prion cseruleus. Blue Petrel. 

 o- 



Procellaria caerulea, Gmel, Syst. Nat. i, p. 560 (1788) ; Layard, B. S. 



Afr. p. 361 (1867). 



Procellaria forsteri, Smith, Illusir. Zool S. Afr. Aves, pi. 53 (1840). 

 Halobsena cserulea, Sharpe, ed. Layard's B. S. Afr. p. 768 (1884) ; Swin- 



burne, Proc. B. Phys. Soc. Edin. ix, p. 198 (1886); Salvin, Cat. 



B. M. xxv, p. 431 (1896) ; Shelley, B. Afr. i, p. 168 (1896). 

 Prion cseruleus, Eeichenow, Vogel Afr. i, p. 31 (1900) ; Vanho-jfen, Joiirn 



Orniih. 1901, p. 310. 



