CHAP. IX.] 



RADUL^E I BUCCINUM. 



263 



*402. The range of Variation may be still greater than this, the 

 number of centrals being sometimes as low as 3. Fig. 72 shews 

 the different conditions found. In it eight varieties are shewn, 



M/vWV. 



IV 



VI 



Fig. 72. Variations in odontophore of Buccinum undatum. 



I. Three centrals (Labrador). II. Four centrals. III. Five centrals, approxi- 

 mately symmetrical bilaterally. IV. Five centrals, not symmetrical ; the two 

 external centrals on one side almost separate, correspond with a bifid denticle on 

 the other side (Labrador). V. Six complete centrals (Labrador). VI. Seven 

 centrals (Lynn). VII. Nine almost distinct centrals. VIII. Eight centrals ; 

 laterals asymmetrical (4 and 5). 



I. II. IV. — VI. from photographs made and kindly lent by Mr A. H. Cooke. 

 III. VII. VIII. after Friele. 



I. II. IV. — VI. being taken from Mr Cooke's specimens, III. VII. 

 and VIII. from Friele's figures. 



As thus seen, in these variations considerable symmetry may 

 be maintained. This symmetry and defmiteness of the varieties 

 in the cases with 3 and 4 centrals is especially noteworthy, in- 

 asmuch as these are abnormal forms and have presumably arisen 

 discontinuously. As also seen in the figure, e.g. IV. and VI. 

 this symmetry is not universal, and may be imperfect. The 

 specimen shewn in VIII. is remarkable for the asymmetry of the 

 lateral plates, which have 4 and 5 denticles respectively. 



In connexion with the subject of symmetrical division interest 

 attaches to cases like that shewn in Fig. 72, IV. in which on the 



