SHUFELDT: OSTEOLOGY OF THE STEGANOPODES 



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side of the corresponding osseous opening of the ear, and the foramen ovale. Pos- 

 teriorly the cranial surface, in so far as the occipital area is concerned, lies in the 

 vertical plane, and is distinctly reniform in outline, bounded all about by a raised 

 osseous ridge, with its convex curve above, and the foramen magnum situated me- 

 siad in the concavity below. The unpierced supraoccipital prominence is fairly 

 well-developed, and occupies the mid-vertical line of this reniform occipital area, 

 extending from the foramen to the limiting curve above. 



The mandible of a Tropic Bird is somewhat acutely V-shaped in form with large 

 and deep articular cups for the quadrates. These cups have large pneumatic foram- 

 ina in their concavities, with a single such opening on the upper side of their short 

 inturned processes. Behind, they are vertically, as well as completely, truncated. 



FIG. 2. Superior view of mandible of Pliacthon xthereus. 

 men in his private collection. 



Outline sketch, natural size, by the author from speci- 



Either rarnal moiety is very deep, and laterally compressed, presenting, well 

 forward of the articular end, a large, irregular ramal vacuity. The dentary portions 

 of the mandible have hardly half the height of the ramal limbs ; they are thickened, 

 and each is deeply grooved for nearly the entire length of its superior margin. 

 Anteriorly, they run very close to each other, and the terminal symphysis is deep, 

 averaging in depth 1.5 cm. in P. xthereus, being nearly 2 cm. in some specimens. 

 In an eye of the species just mentioned I count some 15 osseous sclerotal plates ; they 

 are remarkable for the extensive manner in which they overlap each other, and 

 still more for the fact that the anterior ones have a depth of not more than one 

 third of the posterior ones, a uniform graduation taking place in the intermediate 

 plates, above and below, in this osseous circlet. 



We find, too, an interesting structure in the bony parts of the hyoidmn apparatus 

 of Phaethon. In the adult, the ceratohyals have fused in ossification with the posterior 

 half of the glossohyal to form a peg-like bone, which is longitudinally grooved 

 above and convex below. The first basibranchial is a short, thick piece, expanded 

 in front to form an articular facet, while behind it supports the very minute, though 



