364 PKOF. ST. GEORGE MIVAET ON THE 



THE STERNAL KIBS. 



There are six of these on each side ; and the last does not join the sternum, but joins 

 the postaxial side of the penultimate sternal rib. The fourth is a little more than twice 

 as long as the first ; and the fifth is a little longer. The second, third, and fourth expand 

 distally to a moderate extent from within outwards (Plate LXI. fig. 9). 



THE STERNUM. 



This bone (Plate LXI. figs. 9 & 10) is quite like the corresponding bone of Phala- 

 crocorax, except that there are but four distinct articular surfaces on the pleurosteon, 

 while just postaxiad to the pleurosteon an angle (a) projects outwards, just internal to 

 which passes the fifth sternal rib. This projecting angle distinguishes the sternum of 

 Plotus from that of any other of the Pelecanidae. The keel approaches the postaxial 

 margin of the sternum more nearly than in Phalacrocorax or than in any other of 

 the Pelecanidee ; and the lateral xiphoids are narrower and more pointed. 



Besides the four genera described, the two genera Fregata and Phaeton are usually 

 classed with them to constitute the group of the Steganopodes. But, from the point of 

 view here adopted (that of the postcranial part of the axial skeleton only), I have found 

 it impossible to detect characters which seem to me good and sufficient to unite such 

 Steganopodal group together and at the same time divide them off from other forms. 

 The four genera described, on the other hand, constitute a very natural group (the 

 Pelecanidae), capable of being characterized by a number of common characters drawn 

 from the postcranial part of the axial skeleton, many of which at the same time will 

 probably serve to mark it off from all other groups of birds. 



Fregata and Phaeton agreed to differ from the Pelecanidae as follows : The cervical 

 vertebra are only either twelve or thirteen in number, instead of from fifteen to 

 eighteen ; the cervical and cervico-dorsal vertebra together are only fifteen, instead of 

 from seventeen to twenty, as in the Pelecanidae. There is no distinct sacral vertebra at 

 all, or there is only a rudiment of sacral transverse processes. 



In Fregata and Phaeton, again, the lateral acetabular fossa is enormous, instead of 

 being moderate or small as in the Pelecanidae. The sacro-sciatic foramen is very small 

 and short. In the Pelecanidae, except Phalacrocorax, there are always haemal arches to 

 some of the vertebrae, and in the last-named genus such arches are at least nearly 

 completed ; but in Fregata and Phaeton not only are there none, but no tendency to 

 form haemal arches is exhibited. In the two genera just named the dorsum of the 

 postacetabular part of the ilium is broad and dorsally convex, arching backwards and 

 downwards in a way not found in any of the Pelecanidae. In that family there is 

 always a marked interval between the lumbar and caudal vertebrae, through the non- 

 development of the parapophyses of some of the vertebrae. In Fregata (though not in 



