74 



Mr. II. L. Hawkins on an interesting 



comparison of the plans of the two neighbouring areas with 

 that of the area affected (fig. 1) shows that in all three there 

 are the same number of plates in each column (if the single 

 one is counted as belonging to both columns). Supposing 

 that the single interambulacral only has been resorbed in the 



Fig. 1. 



a. 3 V 



IS 



10 



\Q 



e 1 



* / o ~ 

 o o \ 



/ o o~ 



/ o o"~ 



o o \ * 



* / o 5~ 



o o \Q * o 



- £ -^ (o° s 



o^o \ e o 



o * / o o 



o o \° " o 



_5 / O o 



o o \ ° • o 



o o °/ o • 

 o~ \ o a o 



;* » / °.°" 



Plans of one abnormal and two normal interambulacra 

 Echinus escidentus. 



15 



areas of 



Fig. 2. 



TSTTrnz. 



Part of perignatbic girdle of abnormal Echinus escidentus. 



normal areas, then the abnormal area must have developed 

 two such plates in succession, one having been lost in due 

 course. Such a development might be regarded as arrested 

 (towards Bothriocidaris) or progressive (towards some ad- 

 vanced Spatangoids). In view of the effect of the abnor- 



