Manchester Memoirs, Vol. Iv. (191 1), No. 8. 7 



of the body is inclined downward and backward from the 

 umbo when the shell is not inclined anteriorly. 



This direction of the body leaves the posterior portion 

 of the shell as a " wing " with closely opposed sides. The 

 older individuals do not possess this wing, — the alation 

 decreasing with growth. This disappearance has been 

 said to be due to subsequent anterior development, but 

 four facts point to wearing as the cause. 



Firstly. The post-umbonal dorsal margin shows 

 distinct marks of wearing, the edges being rough 

 and broken. 



Secondly. The ligament becomes more and more 

 prominent with age. Though it does not, of 

 course, occupy its original position, still its in- 

 creasing prominence after the adult form has 

 been reached, shows that wearing is still excessive 

 in that part of the shell. 



Thirdly. The pre- and post-umbonal dorsal edges 

 are no longer in a straight line, but make a 

 distinct angle with each other. 



Fourthly. The post-umbonal lines of growth, if pro- 

 duced upwards, meet the pre-umbonal hinge-line 

 produced backwards, and the enclosed space has 

 the form and position of the " wing " of the 

 younger stages. 



This angle cannot have been formed by excessive 

 anterior growth, because an angle thus formed would be 

 re-entrant. Also the growth lines show posterior growth 

 to be greater than anterior. 



The proof that the original slope of the hinge line is 

 due to tilt and not to growth is also shown by the fact 

 that upward posterior growth would produce a re-entrant 

 angle. 



