AXIAL SKELETON OF THE PELECANID^. 365 



Phaeton), on the other hand, there is no such interruption, the transverse processes 

 being continuously developed throughout these vertebrae. 



But the most marked distinction obtains with respect to the cervical vertebrae. In 

 all the Pelecanidse a sudden change takes place in the form of the cervical vertebree, 

 no vertebra being pressed back at its preaxial end before the eighth or ninth, while the 

 seventh or eighth exhibits, for the first time, serially, a postaxiad forking of its neural 

 arch. In Fregata and Phaeton, on the other hand, the change in the shape of the 

 vertebrae is gradual, and not sudden, the fifth vertebra being already pressed back post- 

 axiad at its preaxial end, while the same vertebra, or even the fourth, has already its 

 neural arch forking postaxiad, and therefore with a deeply concave postaxial margin to 

 its neural arch. Again, in all the Pelecanidae the two ilia meet medianly in front of 

 the acetabula and develop a median dorsal ridge, while in Fregata they do not nearly 

 meet together medianly in that region at all. In Phaeton, though they may appear so 

 to meet, yet they do not do so really, but each ilium has its mediad marginal ridge 

 distant from that of its fellow of the opposite side, though the membrane intervening 

 between the two may become ossified, and so produce a more or less deceptive appear- 

 ance of similarity to the structure of the Pelecanidae in this respect. Finally, in the 

 Pelecanidae there is but a single lateral xiphoid process on each side, while in Phaeton 

 there are two on each side, in addition to the median xiphoid process. 



For all these reasons I think it better to keep the genera Fregata and Phaeton apart, 

 and, confining myself here to noticing their great distinctness (with respect to the axial 

 skeleton) from the Pelecanidae, to reserve any possible consideration of their positive 

 affinities till I come to treat of such group as may appear to exhibit similar cha- 

 racters. 



Dr. Brandt, in his paper before referred to, considers the resemblances and affinities 

 of the Steganopodes amongst themselves and with other bird groups. But he bases his 

 estimates upon comparisons of other parts of their anatomy, scarcely making use of 

 characters drawn from the postcranial part of the axial skeleton. It seems to me 

 that though the four genera described form together a very natural group, yet it is 

 difficult to unite together any two of them to the exclusion of the others. Though in 

 some respects Sula resembles Pelecanus more than the other genera do, yet in other 

 respects Plotus and Pelecanus are most allied ; and while Sula and Phalacrocorax 

 might from some axial characters be associated together, yet in others {e. g. in the 

 number of vertebrae and in the fact that it is the ninth from which its neighbours bend 

 dorsad) Phalacrocorax and Plotus may claim kinship. On the whole Plotus is the most 

 exceptional and difi'erentiated type, and should therefore, I think, form one end of the 

 series, which may be begun with Pelecanus, which in some points, at least, appears the 

 least differentiated and most generalized form. 



The characters of the four genera, and of the family they compose, may stand as 

 follows : — 



3d2 



