MILLPORT MARINE BIOLOGICAL STATION. 261 



Deformation in Echinus esculentus. 



I have found a specimen in which the third ambulacrum is 

 constricted above the equator of the test or shell, so that the 

 fourteenth pair of radial plates (counting from the aboral end) 

 only bear two large spine tubercles, whereas the fourteenth pair 

 in the other ambulacra have four tubercles. The total width of 

 the third ambulacrum at the constriction is 1 cm., and that of 

 the normal rays at the same level 1*5 cm. ; the adjoining inter- 

 ambulacral plates are enlarged, so that in profile the test shows 

 only a very slight flattening in the abnormal ray. At present I 

 cannot hazard an explanation of this interesting variation. 



