428 ON THE PETRELS COLLECTED DURING 



small, and there is a peculiarly formed patagial slip derived from the 

 biceps muscle. 



Zool. Chall. Thus in spite of the general superficial resemblance of the Oceanitidae 

 E t Xf xi V01 56 to ^6 sma ^ er f rms f Procellariidae, with which all ornithologists previous 

 to Garrod had confounded them, the differences between the two families 

 are, it will be seen, numerous and important. The special points of 

 resemblance which the Oceanitidse have with such Procellarian genera as 

 Procellaria and Cymochorea such as the general small size, style of 

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

 arrangement, simple sternum and syrinx (the last three peculiarities 

 being also common to Pelecano'ides) may best be explained by supposing 

 that these small Procellarian forms are on the whole less specialized 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 Proceilariidae and Oceanitidse must have 

 been derived. 



The Oceanitidae are a small and, on the whole, 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 anibiens, 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 *. 



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 nails peculiarly broad and blunt f. 



The Proceilariidae, 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 Oceanitidae. 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 themselves specialized 

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



* To the genus Oceanites belong Thalassidroma gracilis (Elliot, Ibis, 1859, p. 391 

 the type (?) of which, now in the Smithsonian Institution, I have examined) and Tha- 

 lassidroma lineata (Peale, Orn. U.S. Expl. Exped. pi. xxxix. p. 403). Thalassidroma 

 segethi (Philippi and Landbeck, Wiegm. Arch. 1860, p. 282) may be the former bird, 

 or, as suggested by Mr. Salvin (Proc. Zool. Soc. 1878, p. 736), Freyetfa grallaria. 



t Besides Fregefta grallaria and melanogastra there seem to be two other species to 

 be referred here, viz. Procellaria albogularis, Finsch (Proc. Zool. Soc. 1877, p. 722), 

 and Fregetta m&stissima, Salvin (Proc. Zool. Soc. 1879, p. 130). 



