134 MEMOIRS OF THE CARNEGIE MUSEUM 



The orbital cavity itself in Gannets is very deep, the eaves of its roof almost 

 overhanging the jugal bar beneath. Its superior periphery is smooth and rounded. 

 All in front of the rhinal chamber is filled in by the spongy mass formed by the 

 united maxillo-palatines. The hinder portions of these bones are, however, still dis- 

 tinct in S. hassana, and they have all the appearance of these elements as they are 

 found in birds which possess them as concavo-convex plates facing outward. 



The rostrum of the sphenoid is a hollow subcylindrical tube, united above with 

 the interorbital septum. As we proceed anteriorly it becomes more flattened from 

 side to side, and gradually rises upward. At a point about half way between the 

 palatines and cranio-facial hinge it terminates in a process directed forward ; above 

 this is the sharpened ethmoidal margin, nearly perpendicular to the long axis of 

 the skull. Osseous wings to the ethmoid never develop in Sida, not even rudi- 

 mentary traces of them being seen at their customary sites. 



The cranio-facial hinge is exceedingly perfect in its construction, being composed 

 of a thin plate of bone occupying the full width of the skull ; the bones both above 

 and below are separated from each other by a small interval for the entire length of 

 the transverse line constituting the hinge. The part played in the mechanism by 

 the maxillaries has already been described above. 



We find the sphenotic process to be more or less bifid and jutting directly out 

 from the side of the skull ; on the other hand, the mastoidal process is a crest of 

 bone curling forward. Between these two the very wide crotophyte valley 

 is seen. 



The quadrate is a large, massive bone, with its mastoidal head composed of two 

 prominent ellipsoidal trochlese, separated from each other by an intervening notch. 

 Below these the shaft is seen to be rather compressed in an antero-posterior direc- 

 tion, and supports in front at its lower half an unusually formed orbital process. 

 This is a thin, triangular plate of bone placed in the vertical plane, and with its 

 apex directed forward. The pneumatic foramen of the quadrate usually occurs on 

 the posterior aspect of the shaft in most birds, but here it is situated to the inner 

 and lower side of this orbital process. In Sula cyanops I find two others on the an- 

 terior aspect of the bone, one near each articular process. 



The pit for the quadrato-jugal is cylindrical and deep, and a perforation at its 

 bottom may lead into the hollow of the bone. On the posterior aspect of the quad- 

 rate we find an irregular facet for the mandible ; it looks directly to the rear and 

 stands at the head of a longitudinal and deep groove which is found between two 

 similarly placed facets on the foot of the bone. 



Each pterygoid is a trihedral and compressed bone with prominent borders. 



