MONOGRAPH OF THE PETRELS. 



d". Abdomen dark, closely freckled with white irrorata, p. 330 



e". Abdomen sooty brown . . . . . . nigripes, p. 332 



/". Abdomen pure white . . . . . . . . immutabilis, p. 336 



d'. Interscapular region and middle of the back 

 slaty-black. 



g". Bill entirely yellowish horn-colour . . . . melanophrys, p. 339 



h". Bill, culmen, and lower edge of mandible 

 yellowish horn-colour ; sides of bill slaty ; 

 crown of head white . . . . . . bulleri, p. 344 



i". Bill black, the culmen paler ; crown of 



head smoky-grey like the hind neck . . platei, p. 346 



XXIV. — Thalassogeron, Ridgway . 



Type 

 Thalassogeron, Ridgway in Baird, Brew, and Ridgway, Water- 

 Birds N. Amer., II., pp. 345, 357 (1884) ; Salvin, 

 Cat. Birds Brit. Mus., XXV., p. 449 (1896). . . . T. culminatus 



Thalassarche, Forbes (nee Reichenb.), Voy. " Challenger," 

 Zool., IV., pt. XI., p. 57 (1882). 



Key to the Species. 



a. Culmen, in adult, bright yellow ; sides of the bill 

 black. 



a'. Culminicorn rounded posteriorly ; lower edge of 



mandible yellow . . . . . . . . . . culminatus, p. 354 



&'. Culminicorn pointed posteriorly ; base of man- 

 dible yellow, but not the lower edge. 



a". A yellow vertical line at the base of the 



mandible . . . . . . . . . . chlororhynchus, p. 357 



b" . No yellow vertical line at the base of 



mandible . . . . . . . . . . eximius, p. 360 



6. Bill generally pale, the sides not black and the culmen 

 not distinctly yellow. 



c'. Whole head and neck nearly pure white. 



c". Bill very stout ; tarsi and toes longer . . cautus, p. 348 

 d". Bill more slender ; tarsi and toes shorter layardi, p. 353 



LIV 



