_1U 



are exactly those of the first specimen except that the sacral and 

 adjoining regions are one segment further back; the last trunk ver- 

 tebra here is the nineteenth instead of the eighteenth, the sacrum is 

 formed from the twentieth and twenty -first vertebrae, and the first 

 complete haemal arch is on the twenty-fourth. 



Instances of unsymmetrical sacra are known to occur in all the 

 principal groups of Vertebrates in which well developed sacra are pre- 

 sent, and cases very similar to those just given have been described 

 by Camerano ('80, p. 448) and by Howes ('90, p. XVI) in Bombi- 

 uator, and, according to Camerano, by Lataste ('79, p. 49) in Alytes. 

 These cases, however, are among the Anura. In the Urodela few in- 

 stances of any kind of sacral variations seem to have been recorded. 

 Huxley ('75, p. 752), in his description of the sacrum of Menopoma, 

 states that there are two sacral vertebrae, each with a pair of sacral 

 ribs; but the specimen from which his figure was taken was without 

 question abnormal, for, as Lucas ('86, p. 562) subsequently showed, 

 Menopoma ordinarily possesses only a single sacral vertebra. Lucas 

 has also called attention to a second case of sacral variation in Meno- 

 poma, in this instance an unsymmetrical variation involving two 

 sacral vertebrae, a posterior one possessing right and left sacral ribs 

 and an anterior one with only a left sacral rib. This, so far as I am 

 aw^are, is the only recorded instance of an unsymmetrical sacrum among 

 the urodela. 



Variations such as I have pointed out, are not without general 

 significance, and have special importance, I believe, in connection with 

 the problem of homology. Their bearing on this question becomes 

 obvious when one attempts to explain the numerical variation of pre- 

 sacral vertebrae. In such explanations it is customary to proceed 

 upon one or other of the following assumptions: either the sacrum is 

 fixed and the variation is produced by the intercalation or suppression 

 of vertebrae, or the vertebral column is stable and the pelvic girdle 

 travels backward or forward over it. 



The first assumption may be applied in the following way. The 

 peculiar form and position of the first vertebra in any specimen of 

 Necturus is such that one is justified in pointing out its exact homo- 

 logue in any other specimen, and, as we have supposed the sacrum 

 to be fixed, it follows that the variable elements must be looked for 

 somewhere between these two structures, i. e. among the seventeen 

 or eighteen intervening vertebrae. The intercalation of a vertebra in 

 this region would, of course, raise the typical number of seventeen to 

 eighteen; but to assume that such an intercalation has taken place 



