Manchester Memoirs, Vol. Iv. (191 1), No. 15. 3 



The proof of this quincuncial arrangement is obtained 

 by joining up the tubercles next to the marginal carina, 

 then those of the second row, and so on. If, after this 

 has been done, a growth line which cuts a tubercle is 

 traced across the shell, it will be found to cut tubercles on 

 alternate radii only, as in Fig. 2. 



The diagonals do not form straight lines. The devia- 

 tions from straightness occur in two places. 



Firstly. The posterior upper and larger part of the 

 diagonal is slightly bent. This is much less clearly 

 marked in late stages. 



Secondly. Near the anterior edge the lines of ornament 

 bend sharply upward so as to approach a horizontal 

 position. This bend only occurs in those diagonals which 

 reach, or nearly reach, the anterior margin. 



These curves vary in intensity with the individuals. 

 They are most marked in those specimens which are 

 elongated, and laterally compressed, and have the orna- 

 ment carried right up to the anterior margin. 



Relation between Growth and Variations in Ornament. 



The first bend appears to be due to the divergence of 

 the radials. Fig. 3cr represents diagonals of a figure with 

 such radii. As the radials diverge the figure enclosed by 

 them and the concentrics becomes a trapezoid, at the 

 angles of which the tubercles lie, and whose diagonals 

 form the line of ornament. Fig. y gives ABCD and 

 DEFG as such trapezoids. The elongation of the base 

 GF of the figure DEFG forces its diagonal DF out 

 of the line of AD, the diagonal of ABCD. 



This appears to be the cause of the bending of the 

 upper parts of the diagonals of T. clavellata. 



