342 PROF. ST. GEORGE MIVAET ON THE 



The posterior margin of the ischium looks more dorsad, and develops a much smaller, 

 submedian, truncated or pointed ilio-caudal process^. The sacro-iliac ankylosis is rela- 

 tively as well as absolutely shorter. There is a faint indication of an ilio-pectineal 

 process. 



Viewed dorsally, the preacetabular iliac surfaces are more concave and very much 

 narrower, so that the sacro-sciatic foramina come plainly into view. The external margin 

 of each ischium is more concave (Plate LX. fig. 1). 



The shape of the postaxial margin of the pelvis presents a still deeper excavation, 

 the margins of which (antero-posteriorly considered) are on each side first convex 

 mesiad, and then mesially concave. From the second preaxiad fifth of the convexity 

 (which convexity is formed by the ischium) a process (the ilio-caudal spine) projects 

 mesiad and postaxiad (figs. 1, 2, 3, Ic). 



The transverse diameter between the antitrochanteric processes but little exceeds 

 that of the preaxial part of the ilia, while in Pelecanus it is nearly double. 



Ventrally examined, the pelvis of Sula presents some remarkable differences from that 

 of Pelecanus, in spite of the general similarity between the two (Plate LX. fig. 3). 



In the first place the ventral surface of the most preaxial part of each ilium extends 

 much more out laterally beyond the ends of the vertebral transverse processes. There 

 are but two presacral foramina on each side- (fig. 3, s' & s-). 



The renal or lateral acetabular fossae (_/) are relatively, as well as absolutely, smaller. 

 There are but two^ sacral parapophyses on each side ; and behind these there is on each 

 side an elongated fossa produced by the sudden dorsad flexion of the sacrorcaudal 

 parapophyses. Did these processes extend out as in Pelecanus, they would reach 

 almost, if not quite, to the margin of the ilium. The postacetabular parts of the 

 ilia are so narrow that the sacro-sciatic foramina come plainly into view, there being 

 no wide posterior iliac ventral surface as in Pelecanus ; also the surface of the sacro-iliac 

 ankylosis is very small. 



There is only a very small opening on each side dorsad of the expanded ends of the 

 sacral parapophyses ; and this seems only to lead into the substance of the bone ; for 

 there is no supraacetabular fossa. 



There is an indication of an ilio-pectineal process {Ip) ; and the most preaxial part of 

 the obturator-foramen appears from this point of view to be (though it is not) separated 

 off by the here more marked process of the ischium. 



' This process is very prolonged, and pointed in S. australis (1186a), S.fusai'! (1186b), and S.pisMtor 

 (1186c, College of Surgeons); also in a species from Chili (l^o. 779 a in the British Museum). 



^ As has been said, there may be three. 



' There may he three or only one, as before stated. Only in S, eapensis (No. 527 in the British Museum) 

 have I found three such. 



