8 Day, Variation in a Carboniferotis Brachiopod. 



A very interesting point is brought out by Fig. 3 in 

 that, as will be seen from a consideration of the figure, 

 there is a very regular displacement of the maximum or 

 ' mode ' of the curve towards the right, with each increase 

 in length, and this it will be observed is in the direction 



of decreasing — ratio. Since the three curves in order 



from the left correspond to progressive increases in 

 length, then we may conclude that, as the shells increased 

 in size {i.e., throughout life) there was a tendency for the 



value -— to decrease, i.e., for the shell to become broader. 

 B 



The curves obtained for the distribution of — '- in the 



three sets are shown in Fig. 4, and represent the com- 

 ponents of the distribution curve in Fig. 2. Here, as in 

 the curves in Fig. 3, each curve is a regular simple 

 variation curve, and we have again a displacement of the 

 maximum or ' mode' of the curve towards the right, with 

 each increase in length, and this it will be observed from 



the figure is in the direction of decreasing — ratio. Since 



& D 



the three curves in order from the left correspond to pro- 

 gressively equal increases in length, then we may con- 

 clude that in this case, as the shells increased in size 

 (i.e., throughout life) there was a tendency for the value 

 — to decrease, i.e., for the shell to become deeper. 



From the curves in Fig. 3, therefore, it was found that 

 as the shell increases in size, i.e., with increasing age, the 

 ratio value — diminishes, and from those in Fig. 4 that 



there is a similar decrease in the ratio value — . It follows 



therefore that both breadth and depth increase at a greater 

 relative rate throughout life, than does the length. This 

 coincidence must involve some degree of correlation 

 between breadth and depth. 



