294 OSTEOLOGY OF TUBINARES AND STEGANOPODES. 



This hitler lias the usual fonn seen among these cormorant like birds, 

 constituting an arch over the foramen magnum, which occupies the 

 center of a concavity below it. The supra-occipital prominence is here 

 distinguished by a low, smooth, median ridge, which traverses this 

 domelike elevation from the intercrotaphyte line to the superior pe- 

 riphery of the foramen magnum. 



The plane of this latter aperture is about perpendicular to the plane 

 of the basis cranii. In outline the foramen is broadly elliptical, with 

 the short axis transverse. At its lower margin we see a large ellip- 



FlG. 2f. Posterior view of the skull of Sula bassana; mandible removed ; life size. By the author, from 

 the same specimen shown in Fii^s. 24 c t scq. 



soidal condyle, with its short axis at right angles with that of the fora- 

 men. Below this again are the oval openings in the basi-temporal, 

 spokeu of by Parker, with the prominent descending processes of this 

 region Hanking them on either side. 



In form the iuferior mandible is spear-shaped, its sides tapering grad- 

 ually to a sharpened apex. These latter, for the outer aspects of their 

 anterior two thirds, show the same character of venated surface as I 

 described for the superior mandible. Posterior to this, however, as 

 well as the inner ramal aspects, the bone is smooth, having the same 

 appearance as in most birds. 



The symphysis is short and develops a spine behind, which points 

 directly backward aud is in every respect similar to the process in the 

 same place, between the sides of the lower jaw, in Herons and Alba- 

 trosses. Each ramus of this mandible is very thick from side to side, 

 but these parts are hollow, and the bone as a whole is very light, owing 

 to the high state of pneumaticity it enjoys. 



The foramina for the entrance of air to its interior are four in num- 

 ber, two on either limb, one being to the mesial side of the articular 

 cup, and another larger, longitudinally placed, elliptical one just be- 

 yond this concavity on the inner aspect of the ramus near its upper 

 border. The superior side of an articular end has a deep excavation at 

 its center upon which the facets for the quadrate do not encroach, so 

 that, when the jaw is articulated, this pit comes opposite the notch be- 



