5G THE VOYAGE OF H.M.S. CHALLENGE!:. 



Tims in spite of the genera] superficial resemblance of the Oceanitidse to the smaller 

 forms of Procellaxiidae, with which all oruithologists previous to Garrod had confounded 

 them, the differences between the two families are, it will he seen, numerous and 

 important. The special points of resemblance which the Oceanitidse have with such 

 Procellarian genera as Procellaria and Oymochorea — such as the general small size, 

 style of coloration, form of skull, comparative simplicity of the tensor patagii arrange- 

 ment, simple sternum and syrinx (the last three peculiarities being also common to 

 Pelecanoides) — may best be explained by supposing that these small Procellarian forms 

 are on the whole less specialised than the larger ones (Fulmars, Albatrosses, Shearwaters, 

 &c), and so retain more of the characters possessed by the primitive and now extinct com- 

 mon form from which both the Procellariidaj and Oceanitkhe must have been derived. 



The Oceanitidas are a small and, on the wdiole, compact group, with but few differences 

 of importance between the four genera contained in it. Of such differences the most 

 important are the loss of the ambiens, and the very flattened nails and feet of 

 Fregetta ; the lengthening of the foot in Pelagodroma ; and the acquisition of an 

 ocreate tarsus by Fregetta and Oceanites. Garrodia is, therefore, on the whole, the least 

 modified form of the group. The four genera may be distinguished as follows : — 



Garrodia. Ambiens present ; tarsus scutellated anteriorly ; sternum posteriorly 

 entire. 



Oceanites. Ambiens present ; tarsus ocreate ; sternum posteriorly slightly excavated ; 

 interdigital webs yellow. 1 



Pelagodroma. Ambiens present; tarsus scutellated; sternum and webs as in 

 Oceanites; feet very long. 



Fregetta. Ambiens absent; tarsus ocreate; sternum entire; feet very short, and 

 nads peculiarly broad and blunt. 2 



The Procellariidas, comprising as they do by far the greater number of species and 

 genera of the group, show much more divergence inter se than is the case with the 

 Oceanitidas. The Albatrosses are by far the most aberrant forms of this group, with 

 which, however, they have all the characters above noted in common, though in them- 

 selves specialised in several points. The discovery of a rudimentary hallux, and of an 

 aftershaft in these birds, disposes of two of the characters which have hitherto been 

 available for their separation from the other Tubinares, as do the gradations of form 

 that exist in the amount of separation of the two parts of the dorsal tract of another. 

 As peculiarities of the Diomedeinse may be included : — ■ 



1 To the genus Oceanites belong Thalassidroma gracilis (Elliott, litis, 1859, p. 391 — the type (?) of which, now in 

 the Smithsonian Institution, I have examined) and Thalassidroma lineata (Peale, Orn. U.S. Expl. Exped., pi. xxxix. 

 p. 403). Thalassidroma segethi (Philippi and Landbeck, Wiegm. Arch., 18u'0, p. 282) may be the former bird, or, as 

 suggested by Mr. Salvin (Proc. Zool. Soc, 1878, p. 736), Fregetta grallaria. 



Besides Fregetta grallaria and melanogastra there seem to be two other species to be referred here, viz., Procellaria 

 albogidaris, Finsch, (Proc. Zool. Soc, 1877, p. 722), and Fregetta mcestissima, Salvin (Proc. Zool. Soc, 1879, p. 130). 



