82 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
An interesting skeletal variation in one of the specimens of group C 
just described is the occurrence on the left side of the 19th 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 vertebra of Rana, as recorded by 
Bourne (’84, p. 87). 
Theré 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 vertebre. These ribs 
aro both well formed, but the posterior one is much the shorter, and 
from comparison with the other 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 CONSIDERATIONS. 
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? (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 intercalation 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 
