80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
23d vertebra, The topography of the plexus is of the a type. There 
is no asymmetry in its distribution to the appendages, although the 
plexus of the left side must as a whole trend further caudad in order to 
reach the foramina of exit. On the left side the backward trend of xıx 
exposes a longer antero-posterior area of the body wall to which xvir is 
distributed than is exposed on the right side. In accordance with this, 
nerve XVII of the left sido is slightly stronger than its companion of the 
right side. Also nerves xıx and xx of the right side are somewhat 
stronger than their mates of the left side; i. e. the strength-centres of 
the two sides are to a slight extent unsymmetrically placed, but this hy 
no means equals the extent of an entire segment, 
To which group, A or B, is this specimen more closely related? As 
in both these’ groups the first haamal arch may occur (see Table) on 
the same vertebra as in the specimen under consideration, this criterion 
is of no value in determining the affinity of the specimen. But the fact 
that the plexus of this specimen is of the a type indicates that it falls 
under group A, to which this type is restricted (see Table). A glance 
at Figure 5 shows that the main nerve of the plexus is xx, and that xxii 
fails to enter the plexus, both conditions typical of group A. Hence 
I consider this specimen to represent an individual like those of group A, 
in which, however, the sacral rib of the left side is displaced one seg- 
ment caudad, There is not a corresponding displacement through one 
segment of the plexus of the left side, but only a slight displacement of 
the strength-centre caudad. 
The two other unsymmetrical specimens (Plate 2, Fig. 6) have in 
each case the right sacral rib on the 18th vertebra, the left upon the 
19th. In both cases the first heemal arch is on the 22d vertebra. 
The plexus is of the ß type, but with the modifications that in both 
cases the posterior branch of nerve xviir enters strongly into the plexus, 
and further in both cases nerve xx1 does not enter the plexus, but re- 
mains a weak nerve distributed to the wall of the pelvic cavity. The 
plexus is consequently formed of nerves XVIII, xıx, and xx. Nerve xvin 
is stronger than usual, xix equals the strength of xx, and, as noted, xx1 
is very weak, being no stronger than an ordinary spinal nerve, all of 
which shows a displacement of the strength-centre cephalad. I can 
detect no marked want of symmetry in the locus of the strength-centre of 
the two sides, but if there be any, indications are that it is toward a 
yet further displacement cephalad of the plexus of the right side. 
The position of the heemal arch on vertebra 22 and that of the sacral 
ribs remove these two specimens from any relationship to group B, 
and I therefore conclude that they represent individuals like those of 
