Xo. 4. — Variations in the Brachial and Luvibo-Sacral Plexi of 

 Necturns niacalosus Rafinesque} By F, C. AVaite. 



Variations in the position of the pelvic girdle in Vertebrates, espe- 

 cially in Amphibia, have already been noticed by several authors; 

 Adolphi, Bourne, Case, Howes, G. H. Parker, and others. These varia- 

 tions, which obviously involve an inconstancy in the number of presacral 

 vertebrae, are as a rule symmetrical, the entire girdle being one segment 

 caudad — more rarely cephalad — to the usual position ; but infrequent 

 instances are found in which the right and left constituents of the girdle 

 have unsymmetrical positions. 



Closely associated with the pelvic girdle is the lumbo-sacral plexus, 

 and the problem which suggests itself in this connection is to determine 

 whether, with the variations known to occur in the skeletal structures 

 of the girdle, there are correlated variations in the lumbo-sacral plexus. 



If, as some morphologists believe, a difference in the position of the 

 sacrum is the result of increase in the number of vertebra; by splitting 

 of one or more presacral vertebrae, or decrease through fusion, then 

 such phenomena must take place in one or the other, or both, of two 

 regions, either anterior to the most posterior nerve of the brachial 

 plexus, or between that point and the lumbo-sacral plexus. 



To obtain evidence upon these alternatives, 1 dissected out the 

 brachial plexus, in addition to the lumbo-sacral, in all the specimens 

 which I have studied. As this plexus involves less variation than does 

 the lumbo-sacral, I shall discuss it first. 



In naming the spinal nerves, I have adopted the plan of calling that 

 nerve which emerges between the cranium and first vertebra the first 

 nerve,'* succeeding nerves being consecutively numbered. 



So far as I have noticed, there is little variation among either the 

 sympathetic or dorsal branches of the spinal nerves. Since only the 

 ventral branches enter the plexi, I shall, for the sake of brevity, desig- 



^ Contributions from tlie Zooloprical Laboratory of tlie Museum of Comparative 

 Zooloiry at Harvard College, E. L. Mark, Director, No. LXXXV. 



In the Anura, where there is no spinal nerve eraerginpf anterior to the atlas, 

 the sc called first spinal nerve emerges between the first and second vertebrae, and 

 is homologous to the second nerve of Urodela. 



VOL. XXXI. NO. 4. 



