SHITFELDT : THE OSTEOLOGY OF THE STEGANOPODES 119 



■ The cranium is broad and spreading behind, and but moderately narrowed 

 anteriorly, and this form defines the shape of the superocranial aspect of the skull, 

 where the smooth, rolhng frontal portion is succeeded, as we pass backwards, by 

 the flat and wide space that separates the sharp marginal borders of the orbits above. 

 SHght parial elevations characterize the parietal region, and this surface falls away 

 gradually, upon either side, towards the post-frontal processes. Deeply sculpt, the 

 crotaphyte fossae are mesial ly separated by a broad interval, smooth and unmarked. 

 A remarkable feature of the skull of one of these birds, and it is still further 

 enhanced when the lower mandible is articulated, is the appearance it has of having 

 been vertically truncated in the transverse plane or direction, or being cut squarely 

 across, reminding us of the skull in certain Herons and their allies. 



Regarding the skull of a Tropic Bird, upon its lateral aspect, we are at first struck 

 with the large, squarish orbit, bounded below by the straight, stout zygomatic bar ; 

 posteriorly by the massive quadrate, and the spreading post-frontal process ; ante- 

 riorly, by the great, free lacrymal bone, with its projecting upper limb, and its ver- 

 tical portion, which latter is twice perforated posteriorly by conspicuous pneumatic 

 foramina. The interorbital septum invariably presents a large quadrilateral vacuity 

 at its central surface, which is usually distinct from the orbital foramina posterior 

 to it. A mere apology in bone represents the almost thoroughly aborted ethmoidal 

 wing, or pars plana. The foramen for the exit of an olfactory nerve is exceedingly 

 small a-s it occurs upon the anterior wall of the brain-case, and the groove leading 

 from it is open ; but the anterior foramen seen above the serai-aborted pars plana 

 is of considerable size, and is usually completely surrounded by bone. In front, the 

 margin of the mesethmoid is deeply notched, being produced below as a conspic- 

 uous, blunt-pointed process, — just the reverse, for example, of what we find in Sula. 



Turning to the inferior aspect of the skull we meet with many points of interest, 

 and a construction of parts by no means typically steganopodous (see Plate I., Fig. 2). 

 In front the maxillopalatines are distinctly developed, being elegant concavo-convex 

 processes, with their convex sufaces parallel with each other but not in contact 

 mesially. Their concave external aspects are partially filled with a very open can- 

 cellous tissue of bone, and they have a broad base in each case, which appears in 

 the adult skull to coossify with the corresponding nasal, maxillar}^, and perhaps, 

 premaxillary. The postero-inferior angles of the latter are, upon either side, pro- 

 duced backwards as a prominent process which, when the lower mandible is artic- 

 ulated with the skull, outwardly overlaps its margin opposite to them. 



The palatines, although in contact for their entire surfaces in the middle line, do 

 not co5ssify as is the case in the Cormorants, Gannets and other steganopods. 



