Petrels from the Nor(h-East Pacific Ocean. 577 



Our examination of all these birds causes us to differ both 

 as regards the genera and subspecies to be recognised. 

 Neither of the two works quoted admit subspecies at all, 

 the * Monograph ' doubting the validity of some of the 

 forms which in the ' Check-list ' are admitted as species. 



To deal with the genera first. In the ' Monograph ' 

 Procellaria = Hyclruhates=Thalassidroma of the American 

 * Check-list ' is diagnosed as above by means of the pro- 

 portion of the tarsus to the middle claw and toe and 

 presence or absence of a fork in the tail-feathers. Neither 

 of these characters holds good in the species assembled under 

 the genus, for the second species, tethys Bp., has the tail 

 forked. This species is small like H. pelayica,hv\t other- 

 wise is a somewhat typical Cymochorea. Its removal would 

 leave pelagica as the sole representative of Procellaria auct. 

 = Hydrobates = T/talassidroma. 



Oceanodroma should be restricted to the typical species 

 O. furcata Gmelin, as though structurally there is little 

 differentiation between the genotype and other species of 

 Cymochorea, yet, as Cones pointed out, it possesses a radically 

 different style of coloration. There is practically no difference 

 in the coloration of species of Hydrobates, Halocyptena, 

 and Cymochorea, though structurally there is considerable 

 variation. The first-named is a very small bird, with a 

 square tail and very short legs and feet; the second is even 

 smaller, with more delicate legs and feet, but has developed a 

 wedge=shaped tail. The last-named has, if casiro be included, 

 evolved from a somewhat small bird with a tail emarginate 

 only, to a rather large one with a long- forked tail, while it 

 has only diminished in size though also showing a fork in 

 the tail in the case of tethys Bp. The only break in the 

 uniform darkness of the coloration of all these species is the 

 presence of a white rump in some cases. 



In the four genera here indicated the wing formula is the 

 same, viz., the second primary longest, the third very little 

 shorter, the first and fourth subequal, Tiie tarsus is covered 

 with reticulate scales throughout, and the toes have sharp 

 narrow, not flattened, claws. 



