364 PROF. ST. GEORGE MIVART ON THE 
Tue Sternau Rips. 
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). 
Tue STERNUM. 
This bone (Plate LXT. 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 Pelecanide. The keel approaches the postaxia] 
margin of the sternum more nearly than in Phalacrocorar or than in any other of 
the Pelecanide ; and the lateral xiphoids are narrower and more pointed. 
Besides the four genera described, the two genera Fregata and Phaéton 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 
Pelecanide), 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 Phaéton agreed to differ from the Pelecanide as follows :—The cervical 
vertebre are only either twelve or thirteen in number, instead of from fifteen to 
eighteen; the cervical and cervico-dorsal vertebrae together are only fifteen, instead of 
from seventeen to twenty, as in the Pelecanide. There is no distinct sacral vertebra at 
all, or there is only a rudiment of sacral transverse processes. 
In Fregata and Phaéton, again, the lateral acetabular fossa is enormous, instead of 
being moderate or small as in the Pelecanide. ‘The sacro-sciatic foramen is very small 
and short. In the Pelecanide, except Phalacrocoraz, there are always hemal arches to 
some of the vertebrae, and in the last-named genus such arches are at least nearly 
completed; but in Fregata and Phaéton not only are there none, but no tendency to 
form hemal 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 Pelecanide. In that family there is 
always a marked interval between the lumbar and caudal vertebrex, through the non- 
development of the parapophyses of some of the vertebra. In Fregata (though not in 
