870 H. H. SWINNERTON. 
resemblances, at another by the equally great differences. In 
both the girdle and fin together form a continuous whole; 
concavo-convex in shape with the concavity facing inwards. 
In both the fin predominates over the girdle, though here 
this predominance is carried much further. This no doubt is 
associated with the fact that the stickleback begins to lead an 
active free-swimming life at a much earlier pis than does 
the salmon. 
On the other hand in the stickleback there is an 1 entire 
absence of both pre- and meso-coracoid processes. . The 
glenoid line tends to be more vertical. The postcoracoid 
process is enormously developed, being about the same size as 
the whole of the remainder of the girdle. ‘The postcoracoid 
foramen is unrepresented, and the fourth nerve passes down 
to a muscle which runs along by the side of the process. The 
level at which it reaches this muscle is indicated by an 
asterisk (7. 4). 
The common trunk for the first three spinal nerves forks 
at the position indicated (jn. 1—3). 
Stage II (fig. 10)—Owing to the growth of the girdle 
the disproportion between it and the fin-plate has diminished 
considerably. ‘There is still no trace of a division between 
them, and together they are concavo-convex as before. 
‘The fin-plate shows no sign of differentiation into radials. 
‘he scapulo-coracoid region has grown much further 
anteriorly and dorsally, thus bringing about a rotation of the 
glenoid line into an almost vertical position. ‘The pracora- 
coid process has no actual existence, but is represented by 
the antero-ventral angle of the coracoid. 
The postcoracoid process is relatively larger than before 
and has become bent downwards. 
Stage III (fig. 11).—The fin and girdle. still form a con- 
tinuous piece of cartilage without any sign of either foramina 
or radials. It is now much flatter. 
The glenoid line makes about the same angle with the 
vertical as it did in Stage II, but its dorsal end comes much 
nearer to the upper corner of the fin-plate. 
