362 PKOF. ST. GEORGE MIVART ON THE 



The Saceo-caudal Vertebrae. 



These are four in number ; and the first two seem to be devoid of parapophyses, thus 

 making a more or less marked distinction between them and the sacral vertebrse. 

 Compared with those of Phalacrocorax, they are less elongated antero-posteriorly, less 

 laterally compressed as to their centra, with shorter transverse processes. 



The Caudal Vertebra. 



There are seven or eight caudal vertebrae, not counting the pygostyle. They resemble 

 those of Phalacrocorax ; but their parapophyses ai-e not so much bent dorsad, and are 

 more dorsal at their origin in the preaxial vertebrae, descending to a slightly more 

 ventral origin as we proceed postaxiad. 



The PYGOSTYLE is even more compressed laterally than in Phalacrocorax, and thus 

 difiers even more than does the latter from the pygostyle of Sula. On the other hand, 

 it is much more curved, like the claw of a large Fells (Plate LVIII. fig. 47). 



The Pelvis. 



This bony mass includes fifteen or sixteen vertebrae, extending preaxiad over the 

 twenty-fifth (and sometimes even over part of the twenty-fourth) vertebra. It is very 

 much less elongated, relatively as well as absolutely, than in Phalacrocorax, and is 

 shorter, relatively, than even in Sula, so as to be a diminutive representation of that of 

 Pelecanus as to its general proportions, though in certain details it most nearly 

 approaches the pelvis of Phalacrocorax. 



Viewed laterally, the sacro-ischiatic foramen is seen to be less elongated than in Pha- 

 lacrocorax, and relatively broader dorso-ventrally. The union of the ilium and ischium 

 postaxial to it is short, shorter than in any of the before-described genera except in 

 Sula, while the ischium appears relatively deeper dorso-ventrally than in any. The 

 obturator-foramen is also smaller than in any of the other forms. The ilio-caudal 

 spine is long and pointed, as in Phalacrocorax (Plate LXI. figs. 6 & 7). 



Viewed dorsally, the most striking features of the pelvis are the strong, curved 

 posterior iliac ridges which bound the sacral part laterally, which part is relatively 

 wider than in any preceding form save Pelecanus. These ridges absolutely distinguish 

 Plotus from all the other three genera (Plate LXI. figs. 6 & 7, Ir). 



The antero-posterior length of the preacetabular part of the pelvis, as in Phalacro- 

 corax, is much less, compared with that of the postacetabular part, than in Pelecanus 

 or Sula ; and it agrees with that of Phalacrocorax in the wing-like lateral expansion of 

 its most preaxial part, and in the strong median ridge. 



The ilio-caudal spine projects postaxially little, if at all, beyond the ischium; and the 

 angle formed by it with the adjacent margin of the ischium is less acute. 



The external margin of the postaxial half of the ischium is convex, as in Phalacro- 

 corax, not concave as in Sula. 



