80 BULLETIN: MUSEUM OF COMI'ARATIVE 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 xix 

 exposes a louger autero-postei'ior ai-ea of the body wall to which xviii is 

 distributed than is exposed on the right side. In accordance with this, 

 nerve xviii of the left side is slightly stronger than its companion of the 

 right side. Also nerves xix 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 by 

 no means equals the extent of an entire segment. 



To which gi'oup, A or B, is this specimen more closely related"? As 

 in both these groups the first ha,'mal arch may occur (see Table) on 

 the same vertebra as in the specimen under consideration, this criterion 

 is of no value in determining the afl&nity 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 Figui'e 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. G) have in 

 each case the right sacral rib on the 18th vertebra, the left upon the 

 19th. In both cases the first haemal arch is on the 22d vertebra. 

 The plexus is of the y8 type, but with the modifications that in both 

 cases the posterior branch of nerve xviii enters strongly into the plexus, 

 and further in both cases nerve xxi 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, xix, and xx. Xerve xviii 

 is stronger than usual, xix equals the strength of xx, and, as noted, xxi 

 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 hajmal 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 



