237 



Nachdruck verboten. 



The Sacrum of Necturus ^). 



By W. M. Small\vooi>. 

 With one Figure. 



While studying Necturus in the comparative anatomy course this 

 year my attention was directed to an unusual variation in the sacrum 

 by Miss Edith bronson and in a few minutes later to practically the 

 same condition in a second animal by Miss Edith McDowell. Nec- 

 turus has been used by us for the past six years in this same course 

 and the present variation has not been previously noted. 



Barker '96 and Bumpus '97 have both reported on the variation 

 of the sacrum in Necturus, the former studying 27 individuals and the 

 later 100. Later Waite '97 published the results of his observations 

 on this same region. From these three observers it is shown that in 

 the majority of specimens that the sacral vertebra was the 19th but 

 in about 35% <^f the cases there was asymmetry in that the pelvic 

 girdle was attached to the 19th on one side and to the 20th on the 

 other, and in some instances there were two vertebra with sacral ribs 

 on one side. None of these observers report a symmetrical variation 

 of two on each side. 



The present variation consists of two well defined sacral vertebra 

 and ribs sui)porting the pelvis symmetrically disposed, Fig. 1. The 

 19th vertebra bears a pair of long ribs that slant back to articulate 

 with the head of the ilia. The 20th vertebra has a pair of much 

 shorter sacral ribs that extend almost straight out to articulate with 

 the anterior pair and the ilia. When these two pairs of sacral ribs 

 are compared it is seem that the anterior are longer and more at- 

 tenuated than the posterior and this difference is so marked as to be 

 at once noticed. But when the two sacral vertebra are compared 

 there is no noticable difference to be detected, both are similar in 

 shape, size and the place where the sacral ribs articulate. The position 



1) Contributions from the Zoological Laboratory, Syracuse University, 

 C. W. Harqitt, director. 



