136 A COMPARISON OF SOME PECTENS. 



Fig. 3. Comparison of three valves from Tampa (T.) and three from San Diego (S. D.) to show variation in the 

 transverse half-diameter (height of valve). The ratio of height to width (dorso-ventral diameter of the 



three Tampa shells is, in order, &r, the smallest ratio; ~-&r, the modal ratio; ~7^~, the largest ratio. For 



the three San Diego shells the corresponding ratios are : ~^~, ~ZH~> ~Tj~' 

 Fig. 4. Comparison of shells from Tampa (T.) and San Diego (S. D.) with respect to symmetry. 



Tampa. San Diego, 



mm. mm. 



a. Modal condition 31: 29 ; 23: 20 



b. Most symmetrical 31 : 31 23: 22 



c. Least symmetrical 31 : 25 23 :'17 



Fig. 5. Right and left valves from Cold Spring Harbor, Long Island, for comparison. Notice that the San Diego 

 valves are not less unlike those of Tampa than those of Cold Spring Harbor, L. I. 



