1908.] 



AN ABNORMAL ECHINUS. 



653 



The aboral, however, almost reaches the zigzag intra-ambulacral 

 suture. 



Ambulacrum Y (text-fig. 142)is thearea the development of which 

 is incomplete, and here also abnormal plates occur. In series a the 

 twenty-ninth, or last plate of the series, is much deformed, for, 

 while it contains the usual ti-iad, the median demi-plate is pore- 

 less ; while the adoral and aboral primaries are much misshapen, 

 the latter being roughly square and having its pore-pair lying in 



Text-fig. 142. 



Abnoemalities in Ambulaceal Aeeas op ISlehinus esculentus. 

 (For explanation of tlie lettering see text-fig. 138.) 



a peninsula-like corner almost without the boundary of the 

 plate. In series h the penultimate plate, twenty-nine, contains 

 but two elements, an adoral primary and an aboral demi-plate. 

 It is followed by a large rudely-triangular plate whose apex falls 

 at the junction-line of the thirteenth plates of the adjoining 

 interamljulacral areas. Each of the terminal plates of g this 

 aborted area is bounded on two sides instead of on one by inter- 

 ambulacral plates. 



The following table shows at a glance the relative positions of 

 these abnormalities with regard to the respective areas in which 

 they occur : — 



Table III. 



Area 





r 



II 



III 



IV 



^ 



T 









i 







Series 



a 



1 

 49 



i 28 



1 



h 



50 

 29 



a 

 54 



28 



I 



54 



28 



1 a 

 58 



29 



h 



58 



29 



a 

 53 



30 



6 

 53 



29 



a 

 29 



29 



b 

 30 



29 





Total no. of 

 plates. 



No. of first plate 



in each 

 abnormal group. 



Summarising these observations regarding the ambulacral areas, 

 we find that, of the ten rows of ambulacral plates grouped in the 



