WAITE : PLEXI OF NECTUKUS. 



81 



group A, in which the right sacral rib has been displaced cephalad 

 through one complete segment. Superimposed upon this is a dis- 

 placement cephalad of the strength-centre of the plexus through the 

 extent of part of a segment only, as is clearly indicated by failure of 

 XXI to enter the plexus, and the increased strength of xviii. These 

 conditions together represent a state intermediate between that of indi- 

 viduals bearing the girdle symmetrically on the 19th vertebra, and a 

 hypothetical group in which the girdle would be carried symmetrically 

 on the 18th vertebra, and pi'obably with the first haemal arch on the 

 21st vertebra pai't of the time at least. Such a variation has not been 

 described in Necturus, so far as I know, but I think it likely to occur, 

 although the prevailing tendency in abnormal position of the girdle is 

 toward displacement caudad. Indeed, DavidofF ('84, p. 412) has found 

 such symmetrical displacement cephalad in Salamandra maculosa. 



Of the 157 specimens of Necturus recorded by Parker ('96), Burapus 

 ('97), and myself, thirteen (8%) are found to have nnsymmetrical 

 sacra. Of these thirteen, nine have the sacral rib of the left side 

 further cephalad, four that of the right side. It is thus seen that 

 nnsymmetrical sacra are not very rare, and further that either side may 

 be in advance. Of these four specimens with the right side in advance, 

 three, one recorded byBumpus ('97, p. 466) and two by myself, invade 

 the territory of the 18th segment, and these three ar© the only cases 

 out of the 157 in which that segment is invaded. 



By way of summing up the statistics of certain of the conditions 

 described, the following table of the thirty specimens which I have dis- 

 sected is inserted : — 



TABLE. 



