226 ME. W. P. PYCKAFT ON THE MOEPHOLOGY AISD 



undoubtedly a Struthious, because a primitive, character, yet the Rhea aud 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 primiti\ e type, and it would seem that this last character is to be regarded as a piece 

 of additioual 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 : aud the exposure of the whole of the fused neural 

 crest lying between the pre-acetabular ilium. In Podiciphlidw the post-acetabular ilia 

 meet dorsad of the neural crest, whilst in Coly7nbkhi^ they do not rise beyond the level 

 of the base of this crest. 



In the Pygopodes, again, the iscliiadic 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 PalceognatJue. 



It may be, tlae long, narrow synsacrum is primitive, and that this form of synsacrum, 

 which is characteristic only of the Pygopodes (including Hesperornis) amongst the 

 Neoqnathw, 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 P'ygopodes and P'alceo- 

 gnathce is primitive in type. 



In the fli"^htless members of the Palaognathce 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 vertebrse are always present. 



The uiEpyornithidce and Linornithidce, whilst they agree with the remaining Pakeo- 

 gnathce in the great height of the synsacral neural spine, differ, as we shall see, in. 

 one or two important features from the other Palceogiiathce, though these structures 

 are readily traceable back to the more primitive form. 



In Prommi.s 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 ventral border, which caudad sends down u 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 innominale bone forming a vertical blade veiy slightl} hollowed in 

 its middle. The post-ilium is narrow cephalad and tapers rapidly from before back- 



