THE SACRUM OF THE LACERTILIA. 9 1 



Nor do the allies of the lizards, the mosasaurs, aigialosaurs 

 and dolichosaurs, offer any clue to the primitive condition of the 

 sacrum. In the Mosasauria, according to Dr. Williston, there is 

 never any sacrum, but the ilia are attached directly to the trans- 

 verse processes of the vertebra which is either the second or third 

 pygal, at least in one genus (19). 



The Dolichosauria differ from the Mosasauria in that the former 

 possess a sacrum of two vertebrae (20) but so far as I have been 

 able to determine there has never been made out in these animals 

 any sacral ribs. In Adriosaitrus (21) no sacral ribs can be de- 

 tected because the animal is preserved on its back and no attempt 

 has been made, so far as I am aware, to determine the presence 

 or absence of sacral ribs in this specimen. In Acteosanrus (22), 

 however, the sacrum is well exposed but shows no evidences of 

 sacral ribs. In Dolichosaiims (23) Owen says : " The extremities 

 of the sacral pleuropophyses come into contact in the Dolicho- 

 sanrus but do not coalesce." From Owen's figure it is difficult 

 to make out just what the condition is in the sacrum of this form. 

 The artist has certainly drawn sacral ribs in the figure but this 

 may have been due to fracture or to a misconception on the part 

 of the artist. Owen makes no statement of any sacral ribs. 



In the Aigialosauria from the Cretaceous of Lesina the same 

 conditions hold in the sacrum as we have described for the other 

 forms. Gorjanovic-Kramberger (24) figures a skeleton of Aigia- 

 losannis which gives no evidence of any sacral ribs nor does the 

 author mention any ribs as occurring in the specimen. The 

 evidence from Opetiosaurus (25) is purely negative since the 

 sacral region of the specimen was in a very poorly preserved 

 condition. 



It is an interesting question for speculation just why and how 

 such a condition as we have described should obtain in the 

 lizards. It seems most probable that the Lacertilia constitute a 

 branch which came off in pre-Triassic times from some primi- 

 tive diapsid stem in which the sacral ribs were functional and 

 that later the ribs from some unknown cause became atrophied. 



In conclusion I wish to express my sincere thanks to Dr. 

 Frank R. Lillie, under whose direction this work was done, for 

 his kindly interest in my studies and for his advice. To Mr. W. 



