58 THE VOYAGE OF H.M.S. CHALLENGER. 
But Pelecanoides shows marks of bemg in some respects an early form in the simple 
condition of the tensor patagit 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 
Procellariidee, except the Albatrosses, and like all those forms, except the Procellaria- 
eroup, has basipterygoid facets, 
Pelecanoides 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 Procellariinze at a point 
when the latter had already developed that feature, and therefore at a period after the 
ancestor of the Procellaria-group—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, Pelecanoides might well be placed, as many authors 
have done, m 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 Procellaria-group after the 
special ancestor of the latter was developed, I prefer considering it as simply a highly- 
specialised form of the Procellariinz. 
The Procellariinee so defined fall into a number of smaller groups, distinguishable by 
good characters. 
The “ Stormy-Petrels” of the genera Procellaria, Cymochorea, and Halocyptena * 
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 patagy, peculiar skull with no basipterygoid facets or distinct uncinate bone, 
entire posterior sternal margin, and little specialised syrinx. Procellaria has two 
ceca, Cymochorea one only, and Halocyptena, as already mentioned, has them quite 
absent. 
The position of Pelecanoides has already been fully discussed ; it stands quite per se, 
though presumably derived from a stem common to it and the remaining Procellariin, 
which must have diverged from the less specialised one now represented by the 
Procellaria-group. 
Prion (with which Halobena is probably to be associated) represents a third minor 
group, much specialised as regards its peculiarly broad beak with its fringe of lamelle, 
whilst in its tensor patagw 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 lamellee of 
? Oceanodroma also, I have little doubt, belongs to this group. 
