58 THE VOYAGE OF H.M.S. CHALLENGER. 



But Peleeanoides shows marks of being in some respects an early form in the simple 

 condition of the tensor patagii muscle, in its very simple syrinx, and in the general 

 shape of its sternum. It has the characteristic form of biceps muscle found in all the 

 Procellariidse, except the Albatrosses, and like all those forms, except the Procellaria- 

 group, has basipterygoid facets. 



Peleeanoides is thus, as will be seen, a very well-marked form, though it is somewhat 

 difficult to decide as to whether its peculiarities are such as to entitle it to form a 

 separate sub-family by itself. The presence of basipterygoid facets would seem to 

 indicate that it probably diverged from the general stock of the Procellariinaj at a point 

 when the latter had already developed that feature, and therefore at a period after the 

 ancestor of the Procellaria-gvowp — in many ways the least specialised, and therefore pre- 

 sumably more ancient, of the sub-family, and in which there are no such facets — had 

 already acquired its main characters. 



According to modern ideas, the object of a classification is not so much to represent 

 morphological facts as to indicate the phylogenetic relations of the different forms con- 

 cerned. According to the first view, Peleeanoides might well be placed, as many authors 

 have done, in a special group of its own ; but if we admit, as seems on the whole most 

 probable, that it has been derived from the same stock as the Procelko-ia-gtowp after the 

 special ancestor of the latter was developed, I prefer considering it as simply a highly- 

 specialised form of the Procellariime. 



The Procellariinse so defined fall into a number of smaller groups, distinguishable by 

 good characters. 



The "Stormy-Petrels" of the genera Procellaria, Cymochorea, and Halocyptena 1 

 form one such minor group, distinguished by their general small size and coloration, com- 

 paratively long tarsi, nearly single nasal aperture, simple triangular tongue, simple 

 tensor patagii, peculiar skull w T ith no basipterygoid facets or distinct uncinate bone, 

 entire posterior sternal margin, and little specialised syrinx. Procellaria has two 

 cseca, Cijmochorea one only, and Halocyptena, as already mentioned, has them quite 

 absent. 



The position of Peleeanoides has already been fully discussed ; it stands quite per se, 

 though presumably derived from a stem common to it and the remaining Procellariinse, 

 which must have diverged from the less specialised one now represented by the 

 Proce^a n'rt -group. 



Prion (with which JIalolxena is probably to be associated) represents a third minor 

 group, much specialised as regards its peculiarly broad beak with its fringe of lamella?, 

 whilst in its tensor pataejii arrangement and syrinx it is not highly developed. 



The two genera Pagodroma and Daption seem very central as regards their relation- 

 ships, which seem to be with Prion (as indicated chiefly by the rudimentary lamella? of 



1 Oceanodroina also, I have little doubt, belongs to this group. 



