24 



E. C. CASE 



The thirty-first, first presacral (Fig. 20), resembles the preced- 

 ing in the arrangement of the rib. 



As before mentioned the vertebrae are rapidly growing shorter 

 and the last presacral is remarkably shortened. The length of 

 the vertebrae is shown by the measurements given below. 



Length along bottom line of the centrum of the twenty-eighth - 0.022 "" 

 Length along bottom line of the centrum of the twenty-ninth - 0.019 

 Length along bottom line of the centrum of the thirtieth - - - 0.016 

 Length along bottom line of the centrum of the thirty-first - - 0.013 

 Vertical diameter of the thirty-first . . . . - 0.039 



Horizontal diameter of the thirty-first - - . - - 0.039 



Fig. 20. — Sacral vertebrae from the side with the last presacral and the first 

 caudal. % natural size. 



In the specimen as preserved there is quite a space between 

 the first sacral and the last lumbar, and in this space is a rather 

 large intercentrum. It is quite broad antero-posteriorly and 

 only slightly curved. The lower surface is quite rough, but this 

 may in part be due to accidents of preparation. There is no 

 possibility that there was a space between the vertebrae equal to 

 the width of the intercentrum, so that it must have underlain 

 the line of juncture. 



Length of the intercentrum - - - - - - - 0.032 "" 



Width of intercentrum antero-posteriorly . . - . 0.014 



There are three sacrals (Figs. 20 and 21), which differ very 

 markedly from the- last presacral and the first lumbar in that 

 they are of considerable relative length. The ribs are attached 

 to the transverse processes and sides of the centrum; as in the 



