242 



On the Os Pubis in Crocodilia. 



[Dec. 8, 



the presence of the median notch between their anterior corners for 

 the hinder half of the yolk sac and a direction rather less anterior.* 

 In the smallest specimen the element which Rathke terms the pubis 

 does not diverge anteriorly from the ischium to the same extent as in 

 the adult, but the elements hav e^ he usual form and meet in the 

 middle line, and the difference is unaccompanied by any divergence 



Pelvic cartilage of Crocodilus palustris, showing the relations of the pre -pubic 

 plates to each other when one-third developed in the egg. 



of plan in the pelvis. The unsegmented cartilage is a different kind of 

 evidence from the same cartilage divided into elements which become 

 permanent as ossifications. And I hold segmentation which occurs 

 subsequently at a distance from the acetabulum, which isolates a 

 bone from the pubic region of the pelvis and the ilium, to be evidence 

 that the structure so separated is pre-pubic; while the only other 

 segmentation separates the ilium from the bone which supports the 

 pre-pubis. From this it follows that the pubis has never been distinct 

 from the ischium, and is not developed distally in crocodiles. The 

 notch which defines the anterior part of the ischio-pubic bone corre- 

 sponds with the great notch in the acetabulum of the mammalian 

 pelvis, and that notch is situate between the ischium and pubis. The 

 cartilage which completes the anterior margin of the acetabulum 

 seems to me to be inseparable from the ilium ; and to be continuous 

 at its border with the fibrous sheath for the head of the femur. I 

 should attribute its existence to the absence of distal development of 

 the pubis, because this would remove the usual pressure of the bone 

 upon the anterior corner of the acetabulum, which stimulates ossifica- 

 tion of the ilium. 



If the interpretation thus made is morphologically sound, it has an 

 important bearing upon the affinities and classification of the 

 Crocodilia. 



