300 University of California Publications in Zoology  [Vou. 23 
all the specimens were very much undersized. Together with Crago 
mgricauda and C. franciscorum this species is one of the only three 
that were taken at more than three stations in the upper bay, and 
together with these two and Hemigrapsus oregonensis is one of the 
only four taken at any of the upper bay stations lying north of Pinole 
Point. 
In general salinity seems to be the controlling factor in the 
separation of the strictly outside species from those found in or ‘‘in 
and out’’ of the bay, and character of bottom that in the distribution 
of species within the bay. Though so important geographically, the 
effect of temperature on the local distribution of species is not at all 
evident. Depth, likewise, apparently exerts little or no influence 
on the distribution of the survey species: certain species were taken 
only beyond the 30 fathom line and others only in the course of shore 
collecting, but the relation of the former to the salinity outside, and 
the latter to their substratum is too intimate to permit any other 
explanation. That only four decapods are known to occur in the 
upper bay north of Pinole Point is probably due to the low salinity 
there obtaining, for although the annual range of temperature in this 
division is considerable it is almost equally great in the more populous 
but much more saline lower bay. 
