128 



A COMPARISON OF SOME PECTENS FROM THE EAST 



compared with posterior partial length. The general methods of calculating are 

 those laid down in my Statistical Methods (Davenport, '99) and are for the most 

 part based on Pearson's ('94, '95) work. 



i. Variability in the Number of Rays. The distributions of frequencies of ray 

 numbers are given for the right and left valves. In the case of the Tampa lot the 

 right and left valves were from pairs. 



DISTRIBUTIONS OF FREQUENCIES OF GROOVES 

 ON INNER FACE OF SHELL. 



The constants derived from these sedations are : 



From these constants we conclude that, in respect to the rays, the Tampa 

 lot has the greater average number. This greater number of rays of the Tampa 

 lot may be related to temperature or the specific gravity of the sea-water. The 

 Tampa lot has, indeed, the greatest number of rays of any lot I have examined, and 

 it comes from a water with the highest mean annual temperature (about 26 C.) and 

 highest specific gravity (about 1.0270 to 1.0275); whereas the San Diego lot, like 

 that of Cold Spring Harbor, experiences a mean annual temperature of nearer 14 C. 

 and a specific gravity of, say, 1.026. (See Murray, '95, Maps 1, 2.) 



The index of variability () of the rays is slightly less at San Diego than at Tampa, 

 but if the coefficients of variation (c) be compared the differences appear insignificant 

 since they are either little greater or much less than the sum of their probable errors. 

 It seems not unlikely that in a long series of variants, like the rays, the variability 



