Ko. 644] SHORTER ARTICLES AND DISCUSSION 283 



of arrangement they belong. They seem usually to be more like 

 Type II than Type I. The groove widens rapidly, however, and 

 the four characteristic rows are quickly established. In the 

 majority of cases the arrangement is undoubtedly like Type I, 

 and this is obviously the source of the usual text-book description. 



However, in at least nine specimens of the seventeen examined, 

 one or more arms have the Type II arrangement. This may 

 occur in any arm, but in my specimens is most frequent in arm 

 e (five cases). Sometimes the Type II arrangement is estab- 

 lished from the very beginning of any regularity at the base of 

 the arm; in my specimens there were three (probably four) cases 

 of this kind in arm e and one in arm d. In such cases the Type 

 II arrangement may persist throughout the entire length of the 

 arm. More commonly, however, the Type I arrangement is first 

 established and after persisting for a longer or shorter distance 

 abruptly changes to Type II. The number of Type I pairs in 

 such cases seems usually to be small. The transformation is made 

 by a slight irregularity on one side such that two long or two 

 short transverse canals are adjacent — and thereafter the arrange- 

 ment is again entirely regular. Sometimes the region of change 

 is more irregular, but never strikingly so. In one case, in arm 

 a, the arrangement was first like Type I. soon changed to Type 

 II, and in the distal part of the arm changed back again to 

 Type I. In the seventeen specimens examined the Type II 

 arrangement has been found to occur (in some part of the arm) 

 as follows: 



The number of arms with Type II arrangement in any one* 

 individual varies considerably, the results for my specimens 

 being as follows : 



1 arm affected 7 cases 



5 arms affected 1 case 



In two cases, both arm n, Type II was found to occur in re- 

 generating arms, though near the base the Type I arrangement 



