XI] 



OF BIVALVE SHELLS 



565 



formulate a law of acceleration according to which points starting 

 from the origin 0, and moving along radial hnes, would all He, at 

 any future epoch, on an elhpse passing through 0; and this 

 calculation we need not enter into. 



All that we are immediately concerned with is the simple fact 

 that where a velocity, such as our rate of growth, varies with its 

 direction, — varies that is to say as a function of the angular 

 divergence from a certain axis, — then, in a certain simple case, 

 we get lines of growth laid down as a system of coaxial circles, 

 and, when the function is a more complex one, as a system of 

 ellipses or of other more complicated coaxial figures, which figures 

 may or may not be symmetrical on either side of the axis. Among 



Fig. 289. 



our bivalve mollusca we shall find the lines of growth to be 

 approximately circular in, for instance, Anomia ; in Lima (e.g. 

 L. suhauriculata) we have a system of nearly symmetrical ellipses 

 with the vertical axis about twice the transverse ; in Solen pellu- 

 cidus, we have again a system of lines of growth which are not far 

 from being symmetrical ellipses, in which however the transverse 

 is between three and four times as great as the vertical axis. In 

 the great majority of cases, we have a similar phenomenon with 

 the further comphcation of slight, but occasionally very consider- 

 able, lateral asymmetry. 



In certain little Crustacea (of the genus Estheria) the carapace 

 takes the form of a bivalve- shell, closely simulating that of a 



