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THE AMERICAN NATURALIST [Vol. XL 



direction occurs at the lateral and anterior borders of the shell so 

 as to produce maximum growth almost or completely at right 

 angles to the plane of separation of the valves. This change is 

 frequently so great as to produce a reentrant groove of greater 

 or less depth at the junction of the valves, at the lateral and an- 

 terior portions of the shell. The groove results from the failure 

 of each successive growth line to build out as far as the preceding 

 one, and thus results in bending in the edges of the valves so that 

 they meet in a depression. 



Examples: in a pedicle valve of Athyris spirijeroides (Eaton), 

 No. 498, the first lamellose growth lines appear after the shell has 

 attained a length of 22 mm. and a width of 26 mm. At this period 

 in growth the shell not only ceased to increase in width at the 

 cardinal angles but actually decreased and so produced a groove 



1 mm. in depth (Fig. ()). The change in the angle of curvature 

 took place at the anterior portion of tlie valve later than at the 



mm. in length by ^5 mm. in wiMth, \\w change in the angle of 

 curvature in old age at the sides of the shell is ubont i:,°, and at 

 the anterior portion is much less. This specimen also shows a 

 shallow, broad groove at the cardinal angles (Figs. 1 and 2). 

 A slight groove is also developed at the cardinal angles of a 



