8 Mr. E. C. Punnett. On the Composition and Variations 



The range of variation with regard to the following points will be 

 first examined, after which it is proposed to find out whether any 

 correlations between the different parts can be established. 



I. The serial number of the first girdle-piercing nerve. 



II. The number of the post-girdle nerves. 



III. The number of nerves forming the collector. 



IV. Asymmetrical variations. 



V. The number and position of the nerve canals. 



VI. The number of the fin rays. 



VII. The number of the whole-vertebrse. 



I. Variations in the Serial Number of the First Girdle-piercing Nerve. 



According to Davidoff's account the pelvic girdle is pierced by two 

 foramina, through which pass the ventral elements of two consecutive 

 fin nerves. Of these two foramina the more laterally situated is the 

 larger, and affords a passage to the ventral elements of the nervus 

 collector. Variations occur in this arrangement as will be seen below. 

 In this paper the last nerve taking part in the formation of the nervus 

 collector has always been regarded as the " first girdle-piercing nerve," 

 though occasionally, as will appear below, the foramen through which 

 it passes is no longer complete, but is reduced to a groove. The sub- 

 joined table shows the varying position of this nerve in the two sexes. 



Table II. 



Since the serial number of the first girdle-piercing occasionally 

 differs on the two sides of the same individual, each side has been 

 given separately as an individual case. Consequently the 100 cases 

 for each sex in the above table are taken from fifty specimens respec- 

 tively. The chief point of interest here is that the males show a 

 marked tendency to a more rostral position of the girdle, as compared 

 with the females. This amounts to 37 -25 - 36-43 = 0'82, or nearly 

 a whole metamere. 



