WAITE: PLEXI OF NECTURUS. 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 xvm. 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 probably with the first hemal arch on the 
21st vertebra part 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), Bumpus 
(97), and myself, thirteen (8%) are found to have unsymmetrical 
sacra, Of these thirteen, nine have the sacral rib of the left side 
further cephalad, fonr that of the right side. It is thus seen that 
unsymmetrical sacra are not very rare, and further that either side may 
be in advance. Of those four specimens with the right side in advance, 
three, one recorded by Bumpus (’97, p. 466) and two by myself, invade 
the territory of the 18th segment, and these three are 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. 
First Heemal Arch on 
Vertebra which Num. f 
Dario Some ühekimens. a Type. u gene 
Vertebra 22, Vertebra 23. 
19 20 10 10 10 10 
20 i 0 7 0 7 
19 (rt) 
and 20 (lft.) 1 1 0 0 1 
18 (rt.) 
and 19 (Ift.) 2 0 2 2 0 
