140 PROCEEDINGS OP THE AMERICAN ACADEMY. 



a. Eddystone. — This form uuit is, on the whole, the most extreme, 

 being such in eleven out of the thirteen (not always independent) measure- 

 ments. It has the least minimum dorso-ventral diameter, and therefore 

 the greatest ratio of a-p. to d-v. diameter, when the d-v. diameter equals 

 67 mm. It has the smallest ratio of posterior ear length to total hinge 

 length, and consequently it has proportionately the longest anterior ear. 

 Its globosity is high. Its average as well as modal number of rays is 

 least. The index and coefficient of variability are both least. On the 

 other hand it has relatively the longest hinge joint. In the skewness of 

 ray distribution and in the correlation of d-v. diameter and partial trans- 

 verse diameter it is intermediate. And, it may be added, the upper 

 valves are lightest in color. In general, the right valves are the smallest, 

 longest, most globose, and lightest in color of all the lots ; they have the 

 longest hinge line and the beak lies farthest posteriorly on the hinge. 

 The number of rays is smallest and least variable. The d-v. and a-p. 

 diameters are least correlated. 



b. Firth of Forth. — This form unit is least like the Eddystone form 

 unit. The right valves are largest and roundest and flattest of all the 

 lots ; they also have the shortest hinge line. The variability of the rays 

 is greatest, but their skewness least. Their correlations are high. The 

 average and modal number of rays and the position of the beak on the 

 hinge are intermediate. 



c. Irish Sea. — This form unit is, in most respects, intermediate be- 

 tween the others. It has, however, the greatest mean and modal number 

 of rays, with the greatest skewness of their distribution ; also its beak 

 lies farthest forward on the hinge. Its correlations are relatively slight. 



Can we correlate these peculiarities of the form units with any facts of 

 physiography ? The correlation that appears at once is with latitude. 

 Eddystone is at N 50° 15' ; the Irish Sea at 54° 18'; the Firth of Forth 

 at 5G° 5'. The shells occurrin£f at extremes of latitude are extreme in 

 qualities also ; and difference of latitude means difference of tempera- 

 ture. Thus the mean annual marine isotherm of 57° F. for tlie 

 surface in September runs through the Irish Sea near the Isle of Man ; 

 it runs 500 kilometres south of the Firth of Forth in the North Sea and 

 300 kilometres north of Land's End. Clearly, then, the sea temperature 

 of the Irish Sea is intermediate between that of the Firth of Forth and 

 that of Eddystone. Likewise in density of water and depth the Irish 

 Sea locality is intermediate between the other two. 



Again, regarding Pecten opercularis, which is closely allied to P. 

 irradians of the western Atlantic and P. ventricosus of the eastern 



