118 



MEMOIRS OF THE CARNEGIE MUSEUM 



the skull has an average length of about 10.8 centimeters, while in the other form, 

 P. flavirostris, it equals for the same measurement but 9.2 centimeters. Measuring 

 from the mid-point of the cranio-facial hinge, we find that in the yellow-billed 

 species the length of the supero-mandibular portion of the skull exactly equals in 

 length the cranial part, while in P. xthereus the facial portion slightly exceeds the 

 cranial. Viewed from above (Plate I., Fig. 3), it will be seen that the supero-man- 

 dibular moiety is distinctly marked off from the cranium by the ver}^ distinct trans- 

 verse cleft forming the cranio-facial hinge, which latter admits of considerable 

 mobility in the dried skull. The upper osseous bill is broad and massive at its 

 base, but almost immediately tapers as we proceed anteriorly towards its acute 

 apex. The culmen is slightly decurved, while below the latero-external margins 

 are cultrate. Just within these there runs a furrow nearly to the apex, which is 

 bounded mesially by a raised bony ridge. Internal to this again, the central 

 portion, is excavated, from the maxillo-palatines to the apex, being broad posteri- 

 orly and gradually tapering to the front. A delicate medio-longitudinal ridge 



FIG. 1. Right lateral view of the skull of Phaethon lethereus. Natural size. Drawn by the author from a speci- 

 men in his own collection. 



marks the anterior half of this space, which is about equally well marked in 

 the two species. In these birds there is always found a small foramen, perforating 

 the mandible upon either side, just anterior to the lateral terminations of the 

 cranio-facial cleft ; while beyond these, the narial apertures, one upon either aspect 

 of this osseous beak, are seen to be of no great size, being in form elongated, 

 ellipsoidal, communicating, and with smooth, rounded margins. Minute marginal 

 and culminar foramina occur in this part of the skull, while the intervening surface, 

 or what is really the sides of the upper osseous mandible, is delicately scrolled over 

 with fine anastomosing venations (Fig. 1). 



The rhinal chamber is somewhat capacious, and the hollow part of this region, 

 clear to the apex, is more or less filled in with bony tissue of the cancellous variety. 



