132 MEMOIRS OP THE CARNEGIE MUSEUM 



A proportionate increase in their length and caliber seems to be the only im- 

 portant difference that distinguishes the skeletal toe-joints of P. sethereus from the 

 corresponding bones in P. flavirostris. To show the relative lengths of the pha- 

 langes in the last named species, see the above table, in which the measurements 

 are given in millimeters. 



Beyond the patella, already described above, I find no other sesamoids associated 

 with the bones of the pelvic limb in the Phaethontidas. 



OSTEOLOGY OF THE 



(See Plates XXII. and XXIII., Figs. 7-12.) 



Of the Skull. Sula bassaua has a skull that averages in the adult about 186 mm. 

 in length ; whereas in Sula piscator 129 mm. is the average length of the skull in 

 the adult. Between these extremes we find the other species to fall, and a simliar 

 variation is size is, of course, applicable to the remainder of the skeleton in these 

 different specific forms of the Sulidae. Apart from this, the characters presented in 

 the skeletons of these birds in the main agree very well indeed, though differences 

 do exist, and these stamp the skeleton of each species with an individuality peculiarly 

 its own. The more important of these differences will be noted as we proceed. 

 (See Plates XXII., XXIII., Figs. 7-11.) 



In form, the superior osseous mandible is flat upon its under side with cultrate 

 tomia, while superiorly it is convex from side to side, and tapers from base to apex 

 gradually to a point, being a little decurved near the extremity. Sometimes we 

 find it pierced by a foramen on this upper side, which leads to its hollow interior, 

 but Sula is without nostrils, though did they exist, their position would perhaps be 

 indicated by the posterior end of the longitudinal furrow that marks the mandible 

 upon its lateral aspect. 9 



An osseous, thoroughly adherent crust, appears to overlay the greater part of the 

 superior surface, the only smooth place being a small area in front of the cranio- 

 facial hinge, and even this is absent in Sula piscator and other species. Its entire 

 surface is marked all over by an exquisite anastomosing venation, the ramifications 



8 Besides some four fossil forms, Dr. R. Bowdler Sharpe in his recent Hand List of Birds, recognizes the following 

 species of the genus Sula, in which are included all the birds of this group at present known to ornithologists : viz : 

 S. basxana, S. serrator, 8. capenxin, S. cyanops, S. abnoti, S. piscatrix, S. webuleri, S. variegata. S. nebouxi, S. sula and S. 

 brewsleri. In the A. O. U. ''Check-List" we also finds, gossi, which may correspond with one of the species above 

 enumerated, as S. nebouxi, or S. websteri. 



9 In my memoir entitled " Observations upon the Osteology of the Orders Tubinares and Steganopodes (Proc. U. 

 S. Nat. Mus. Vol., XL, 1888, pp. 253-315), I give figures of all the principal bones of the skeleton of Sula bassana, drawn 

 by me, natural size, direct from the specimens. These will not be reproduced here, while the osteology of other species 

 of Gannets will be illustrated in the plates of the present memoir, as S. gossi and S. brewsteri. 



