gO ROY L. MOODIE. 



caudal ribs, recalling, as it does, the condition which exists in 

 the modern tailed amphibians {Menopoma, Necturus). McGregor 

 (15) has described separate caudal ribs in the Phytosauria and 

 also describes free sacral ribs for these animals. In the young 

 specimen of Chelydra referred to above the caudal ribs are clearly 

 distinct and are separated from the centra by sutures. In the 

 Ichthyosauria (16) the ribs were free throughout the entire length 

 of the vertebral column. Dr. Williston tells me that he has found 

 free caudal ribs in certain plesiosaurs. In the plesiosaurs, also, 

 Dr. Williston has recently discovered free sacral ribs. From the 

 above enumeration it is clear that caudal ribs are not rare among 

 the reptiles and there can be no doubt, it seems to me, that when 

 a free structure occurs in the sacrum it can be readily homolo- 

 gized with both the presacral and postsacral ribs. In the primi- 

 tive condition the ribs were not differentiated into dorsals, sacrals 

 and caudals and they varied but little in size. The dorsal and 

 sacral ribs are retained in the majority of reptiles having become 

 functional through use or other cause while the caudal ribs which 

 had no real function to perform have become atrophied or vesti- 

 gial. Sacral ribs without doubt exist in the dinosaurs. One 

 striking peculiarity of the sauropod dinosaur sacrum is the elon- 

 gate character of the diapophyses which in many cases serve to 

 aid the sacral ribs in the support of the ilium. This condition 

 obtains in the sacra of Apatosaurus and Brachiosanrus at least. 

 The presence of these diapophyses led Hatcher, without doubt, 

 to contend that the sacral ribs were parapophyses. But so far 

 as I can see the presence or absence of diapophyses could have no 

 effect whatever on the character of the sacral ribs. 



Paleontology helps us not at all in determining the primitive 

 condition of the modern terrestrial lizard sacrum. Of Paligitana 

 (17) from the Triassic of South Africa only a fragmentary skull 

 is known and from this form in the Trias to Iguanavus (18) in 

 the Laramie Cretaceous, a period representing a lapse of millions 

 of years, our knowledge of the terrestrial lizards is a complete 

 blank. Tertiary lizards are represented for the most part by 

 very fragmentary remains and belong, according to our best 

 authorities, to existing families or to families only recently extinct 

 so that they offer no differences in the morphology of the sacrum 

 from the existing forms. 



