226 MR. W. P. PYCRAFT ON THE MORPHOLOGY AND 
undoubtedly a Struthious, because a primitive, character, yet the Rhea and the 
Cassowary contradict this. 
The long and narrow synsacrum, with its high neural spines, finds its counterpart in 
the synsacrum of the Pygopodes ; and this is a point fraught with some significance to 
this last group, inasmuch as we have already evidence to show that the Pygopodes are 
a primitive type, and it would seem that this last character is to be regarded as a piece 
of additional evidence to this end. 
The synsacrum, however, of the Pygopodes differs from that of any Struthious bird 
in (1) its much greater lateral compression ; (2) the absence of any pleurosteal elements 
caudad of the last thoracic vertebra; and the exposure of the whole of the fused neural 
crest lying between the pre-acetabular ilium. In Podicipidide the post-acetabular ilia 
meet dorsad of the neural crest, whilst in Colymbidw they do not rise beyond the level 
of the base of this crest. 
In the Pygopodes, again, the ischiadic fissure of the post-acetabular ilium is reduced 
to a comparatively small foramen, and the post- greatly exceed the pre-acetabular ilia 
in length. 
In the elongated synsacrum, the vertical ilia, and the comparatively high neural 
spines of the synsacrum the Podicipides resemble the flightless Pal@ognathe. 
It may be, the long, narrow synsacrum is primitive, and that this form of synsacrum, 
which is characteristic only of the Pyyopodes (including Hesperornis) amongst the 
Neognathw, was derived from a primitive ancestor which had not yet adopted the 
more usual Neognathine form of pelvis. The fact that the much more recent and 
highly specialized Auks, which have adopted the same mode of life as the ancient 
Divers, have a distinctly Neognathine type of pelvis with widely separated post- 
acetabular ilia supports the view that the synsacrum of the Pygopodes and Palwo- 
gnathe is primitiverin type. 
In the flightless membeys, of the Palwognathe the pre-ilium is always vertical in 
position, and always meets its fellow in the middle line above the synsacrum. 
Pleurosteal elements representing the sacral vertebree are always present. 
The Apyornithide and Dinornithide, whilst they agree with the remaining Palwo- 
gnathe in the great height of the synsacral neural spine, differ, as we shall see, in 
one or two important features from the other Palwognatha, though these structures 
are readily traceable back to the more primitive form, 
In Dromeus the pre- and post-acetabular ilia are subequal in length, or the post- 
acetabular is markedly the longest. The pre-acetabular ilium has a strongly arched 
dorsal border, a sigmoidally curved yentral border, which caudad sends down a long 
acetabular process to join the pubis and serves at the same time as the anterior boundary 
of the acetabulum. ‘The depth from the dorsal to the ventral border is considerable, 
this portion of the innominate bone forming a vertical blade very slightly hollowed in 
its middle. The post-ilium is narrow cephalad and tapers rapidly from before back- 
