PACIFIC FRESH WATER FISHES. 25 1 



ing a perfectly intergrading series from the plateless weak 

 spined williamsoni to the fully plated, strongly armed 

 form of microcephalics. 



Other differences are as follows : 



The tail of microcephalics, especially the more fully 

 plated form, is somewhat depressed and rather slender. 

 In the Colton and Riverside specimens (true william- 

 soni), the tail is heavy, and it is somewhat compressed, or 

 rounded. However this character shows great individual 

 variation, and the well plated form of micro cephalns dif- 

 fers as much from the poorly plated form as the latter 

 does from williamsoni. 



In the well plated forms the tail is provided with a keel 

 on each side. This keel consists of a fleshy basis covered 

 by the lateral plates, which here become keel-shaped. In 

 the poorly plated form and in williamsoni the keel is ab- 

 sent. 



The sides of the thoracic area are decidedly convex in 

 williamsoni, becoming almost straight in the fully plated 

 form of microcepkalus. The post pectoral plate is heavier 

 and notched in the more completely plated specimens. 

 The pubic plate is more sharply pointed and more firmly 

 attached to the interclavicles when the lateral armor is 

 more complete. The first dorsal plates, those in front of 

 the first dorsal spine, are small or wanting in the naked 

 specimens, becoming larger and more firmly attached as 

 the lateral armature becomes more completely developed. 



The specimens of ■williamsoni that I have seen are 

 smaller than the average of microcepkalus. 



There are fully intergrading series in these variations 

 also. 



The following is a summary of the points in which 

 variations exist and which may be used in distinguishing 

 the two varieties. The average representative of the 



