246 CALIFORNIA ACADEMY OF SCIENCES. 



imbricated plates, which cover the whole side of the 

 body anteriorly, becoming smaller posteriorly and form- 

 ing a keel on the tail. So far as is known, Gasterosteus 

 cataphr actus is invariably fully plated from head to caudal 

 fin. This species does not enter the streams. The typ- 

 ical microcephalics has the sides partly plated, and william- 

 soni has been described as entirely without plates. Both 

 of these forms are chiefly confined to fresh water. It is 

 of the variations in these two nominal species — Gasteros- 

 teus microcephalics and Gasterosteus williamsoni — that the 

 present paper treats. 



It was first noticed that the number of lateral plates in 

 specimens of Gasterosteus nu'erocephalus from San Fran- 

 cisquito Creek, which empties into San Francisco Bay at 

 Palo Alto, was considerably less than that of specimens 

 from San Gregorio Creek, which flows directly into the 

 ocean on the opposite side of the peninsula. This led 

 me to the inspection of specimens from other localities. 

 In all, over twenty-two hundred specimens, representing 

 thirty-nine localities, have been examined by me. Spec- 

 imens from localities in the vicinity of Stanford Univer- 

 sity have been collected by various expeditions of the 

 Department of Zoology. Mr. John M. Stowell collected 

 in the San Miguel and San Luis Obispo creeks, Dr. 

 Charles H. Gilbert and Dr. Wilbur W. Thoburn col- 

 lected specimens in southern California. The Carmel 

 River Expedition (N. B. Scofield, C. Rutter, A. Seale 

 and C. J. Pierson) obtained specimens from the coastwise 

 streams in the vicinity of Monterey Bay; specimens from 

 Oregon and Washington have been secured by two ex- 

 peditions of the U. S. Fish Commission; and Mr. E. C. 

 Starks has furnished specimens from the vicinity of Seat- 

 tle, Washington. 



The chief variation is in the number of lateral plates. 



