82 BULLETIN : MUSEUM OF COMPAKATIYE ZOOLOGY. 



An interesting skeletal variation in one of the specimens of group C 

 just described is the occurrence on the left side of the 19tli vertebra of 

 a bifurcate transverse process and a partially double sacral rib (Plate 2, 

 Fig. 7). The two sacral ribs are not distinct throughout their course, 

 but articulate independently with the transverse processes, and normally 

 with the ilium. This condition is parallel with the occurrence of bifur- 

 cated transverse processes on the vertebrae of E,ana, as recorded by 

 Bourne ('84, p. 87). 



There has lately come into my possession a skeleton of this species, 

 the soft parts of which had already been to a considerable extent re- 

 moved. This skeleton shows (Plate 2, Fig. 8) a single sacral rib on the 

 left side borne on the 19th vertebra, while on the right side are two 

 sacral ribs borne one each on the 19th and 20th vertebrae. These ribs 

 are both well formed, but the posterior one is much the shorter, and 

 from comparison with the otlier side, is evidently the supernumerary 

 rib. Each articulates independently with the head of the ilium, and all 

 the joints work easily. The transverse process on the left side of the 

 20th vertebra shows no trace of a rib or articulation, and as it was well 

 covered with the musculature, which had not been disturbed when I 

 received it, I am certain that none existed there. The condition of the 

 specimen when received was such as not to allow determination of the 

 nerve relations. 



Theoretic Consideration's. 



The variations which have been described in the preceding pages 

 involve at least two questions : (1) Does the abnormal position of the 

 girdle arise by intercalation or excalation of presacral segments, by slip- 

 ping of the girdle upon the column cephalad or caudad, or by some other 

 means 1 (2) Is there any correlation between the variations of the plexus 

 and those of the girdle, and if so, of what sort is it % 



The first of these questions is far reaching, and this paper does not 

 aim at an exhaustive discussion of it. It is commonly held that nerves 

 are less subject to variation than either muscles or skeletal parts, and 

 thus serve as a surer basis for homology ; yet this basis is in a degree 

 unstable, for there are certainly considerable variations amongst nerves, 

 as I have shown, for instance, in the lumbo-sacral plexus. 



The evidence for intei'calation or excalation as an explanation of the 

 changes in the presacral length of the column is of a diversified nature, 

 and the opinions concerning such a process may be roughly grouped in 

 two categories : first, that the change in number of segments is due to an 



