PACIFIC FRESH WATER FISHES. 247 



When only a few plates are present, I have always found 

 them to be those which join the post-pectoral plate below 

 and the dorsal plates above. The post- pectoral plate, 

 which is never wanting, is a wing of the pubic plate ex- 

 tending up the side behind the pectoral fin. The first 

 plates to disappear are those in front of the caudal keel, 

 the keel being next to go. Several specimens from 

 brackish pools near Ballard, Washington, are an excep- 

 tion to this, the plates of the caudal keel being the only 

 ones absent. 



Below is given a table showing the variation in the 

 number of lateral plates in specimens from different local- 

 ities, the arrangement being in the order of the average 

 number of plates. I here regard the whole series as 

 forming one species, Gasterosteus williamsoni, with two 

 varieties or subspecies, williamsoni and microcephalics, 

 the latter being the prevalent form; the former, or typ- 

 ical form (because earliest known), originally described 

 from near Saugus in Ventura county, Cal., being more 

 local and confined to inland waters. 



