Vol. 5, p. 261-262. Apkil 15, 1930 



Occasional Papers 



OF THE 



Boston Society of Natural History. 



TWO UNDESCRIBED RACES OF PHAETHON 

 MTHEREUS. 



BY JAMES L. PETERS. 



Comparison of the series of Red-billed Tropic Birds in the 

 Museum of Comparative Zoology with a series of six topotypical 

 examples from Bosun Bird Islet, Ascension Island, reveals the 

 presence of two hitherto undescribed subspecies of this bird. It 

 gives me great pleasure to express my thanks to the authorities 

 of the Cleveland Museum of Natural History for the loan of the 

 series from Ascension Island. 



Phaethon aether eus mesonauta, subsp. nov. 



Type. — Museum of Comparative Zoology no. 238017; adult female, collected 

 at Swan Key, Almirante Bay, Panama, June 3, 1927, by H. Wedel (orig. no. 

 51). 



Subspecific Characters.— Similar to Phaethon cethereus cethereus Linne 1 of 

 Ascension Island, but the greater primary coverts deep black, sometimes 

 tipped, but never edged with white. In the typical form the greater primary 

 coverts present a more frosted appearance and are always broadly edged with 

 white. 



Phaethon aethereus limatus, subsp. nov. 



Type. — Museum of Comparative Zoology no. 65699; adult male, collected 

 at Tower Island, Galapagos Islands, September 3, 1891, by George Baur. 



Subspecific Characters. — Similar to P. a. mesonauta but differing from that 

 form as well as from typical osthereus in having a longer and more slender bill 

 (67-68 against 60-64 mm.) which in dried skins is yellowish horn color, orange 

 red only on the distal third of the culmen. 



Remarks. — I am convinced that the color of the bill in the 

 Tower Island Tropic Bird is of taxonomic value, and is not to be 

 attributed to age, season, or post-mortem change. In a series of 

 twenty-nine specimens of Phaethon sethereus the only two that 

 do not have red bills are two males from Tower Island, which 

 have that organ more slender than any other examples examined. 

 The longest central tail feather of the type of limatus exceeds the 



1 Phaethon cethereus Linne, Syst. Nat., 1758, 10th ed., 1, p. 134. 



261 



