D6 THE VOYAGE OF H.M.S. CHALLENGER. 
Thus in spite of the general superficial resemblance of the Oceanitidee to the smaller 
forms of Procellariidee, 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 Oceanitidee 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 patagi arrange- 
ment, simple sternum and syrinx (the last three peculiarities bemg 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 Procellarudee and Oceanitidee must have been derived. 
The Oceanitidee 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 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.! 
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.’ 
The Procellariidze, 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 
Oceanitidee. 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 Diomedeinee may be included :— 
. To the genus Oceanites belong Thalassidroma gracilis (Elliott, Ibis, 1859, p. 391—the type (?) of which, now in 
the Smithsonian Institution, I have examined) and Thalassidroma lineata (Peale, Orn. U.S. Expl. Exped., pl. xxxix. 
p. 403). Thalassidroma segethi (Philippi and Landbeck, Wiegm. Arch., 1860, p. 282) may he the former bird, or, as 
suggested by Mr. Salvin (Proe. Zool. Soc., 1878, p. 786), Fregetta grallaria. 
* Besides Fregetta 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 mastissima, Salvin (Proc. Zool. Soc., 1879, p. 130). 
