836 



THE EQUIANGULAR SPIRAL 



[CH. 



magnitudes Oa, Oh ... OY, and again on to Oa' , we obtain a diagram 

 such as follows in Fig. 413: by help of which we not only see 

 more clearly the way in which the growth-rate varies from point 

 to point, but we also recognise better than before the nature of the 

 law which governs this variation in the different species. 



Furthermore, the young shell having become differentiated into 

 a dorsal and a ventral part, marked off from one another by a lateral 

 edge or keel, and the inequality of growth being such as to cause 

 each portion to increase most rapidly in the median line, it follows 

 that the entire shell will appear to have been split into a dorsal 

 and a ventral plate, both connected with, and projecting from, 

 what remains of the original undivided cone. Putting the same 

 thing in other words, we may say that the generating figure^ which 



Fig. 414. Development of the shell of Hyalaea (Cavolinia) tridentata Forskal: 

 the earlier stages being the ''' Pleuropits longifiUs'' of Troschel. After Tesch. 



lay at first in a plane perpendicular to the axis of the cone, has 

 now, by unequal growth, been sharply bent or folded, so as to lie 

 approximately in two planes, parallel to .the anterior and posterior 

 faces of the cone. We have only to imagine the apical connecting 

 portion to be further reduced, and finally to disappear or rupture, 

 and we should have a bivalve shell developed out- of the original 

 simple cone. 



In its outer and growing portion, the shell of our Pteropod now 

 consists of two parts which, though still connected together at the 

 apex, may be treated as growing practically independently. The 

 shell is no longer a simple tube, or simple cone, in which regular 

 inequalitiies of growth will lead to the development of a spiral ; and 

 this for the simple reason that we have now two opposite maxima 



