112 THE NAUTILUS. 



Changes with Growth. 



While the morphology of the adult shells has been studied, 

 and also the formation of the young, the intermediate stages 

 and changes have been somewhat neglected. Yet they are 

 essential for a real understanding of at least some of the 

 features of the full-grown. The growth of each valve at the 

 periphery is accompanied by a gradual turning outward 

 around the axis in the ligament, to an angle of about 80 to 

 120 degrees in the mature mussel, or twice as much for the 

 two. With this turning the "teeth" have to keep step, grow- 

 ing, in order to maintain their contact and interlocking. 

 Each cardinal thus forms a widening spiral, though somewhat 

 modified by growth not only towards the opposite valve and 

 cardinal but also upward. But at the same time its base is 

 lengthening, and in most of the species the anterior and 

 posterior ends grow more or less downward over the thicken- 

 ing or widening hinge-plate, and it thus becomes curved or 

 angular. It has been noted before that in some species C3 

 and C2 remain straight or nearly so, and these grow out 

 mostly at their anterior ends. With C4, growth is somewhat 

 different, except for the last-mentioned forms: it is mostly 

 the posterior or lower part which grows, and obliquely down- 

 ward. Of the posterior part of C3, the two components may 

 remain closely connate, as in P. compressum, variabile, cruci- 

 atum etc., or they grow more and more apart. 



There is another element which affects the configuration of 

 the cardinals. With growth and the turning of the valves, 

 the hinge-plate grows, as in length, so in thickness. But the 

 increase in thickness is slight in some species and forms, and 

 in these the cardinals generally stand out free, e. g. splendi- 

 dulum, tenuissimum etc. In others the hinge-plate becomes 

 much thicker, broader in lateral aspect, and it grows around 

 the cardinals so that they are projecting only slightly, or 

 partially not at all ; these again are : P. compressum, supinum, 

 kirkla-ndi, fallax etc., and cequilaterale, varicibile, pauperculum 

 etc., in which C3i and C3o are connate in a compact mass, 

 as there is no space to expand in. There is a well-confined, 



