416 STRUCTUEE AND CLASSIFICATION OF BIRDS 



group, show, as is indicated in the above table, a considerable 

 amount of variation among themselves. And, what is even 

 more striking, the genera vary more than genera as a rule do 

 — for example, the stomach of the darters, the biceps and 

 carotid of Sula, and the muscle formula of the leg of Phala- 

 crocorax. The accompanying table gives some of the chief 

 characters of the several genera. It is interesting to observe 

 from this table that there is to some extent a linear 

 series to be deduced from the facts of structure. If we 

 commence with Phalacrocorax, we may associate with it 

 certainly Plotus and also Sula. In these genera the peculiar 

 fiat and largely fused palatines are associated with the 

 muscle formula AX + . Sula offers a slight step in the 

 direction of Pelecanus. There is a trace of the dorsal ridge 

 of the palatine, and of the anterior vacuity in the interorbital 

 septum for the reception of this edge. In Pelecanus these 

 characters are fully realised, and at the same time the 

 ambiens has vanished. Fregata, which stands apart in the 

 structure of its skull, is also unique in the group by reason 

 of its muscle formula, which is only A + . But this genus 

 and Pelecanus have lost the biceps slip, present in those 

 referred to before, but commencing to disappear in Sula. 



The question is. In what direction has the modification 

 gone ? Are we to start with Phalacrocorax or with Pelecanus, 

 or with some other genus ? 



The answer to this question naturally depends upon the 

 relationship of the Steganopodes to other groups. The group 

 to which they are most nearly allied appears to me (as to 

 Forbes and others) to be the Tubinares. In estimating this 

 affinity Forbes did not, in my opinion, lay sufficient stress 

 upon the ' os uncinatum.' It is true that — as he states — 

 this bone appears also to be present in Chunga and in the 

 touracos, not to mention other birds ; but in Chunga it is a 

 continuation of the descending process of the lacrymal, and 

 articulates with the jugal bar, while in Corythaix it articu- 

 lates both with ectethmoid and lacrymal, though it ends, as 

 in the birds under consideration, on the palatine. In both 

 Fregata and Diomedea this bone connects the lacrymal with 



