The phenomena of ‘plate-crushing’ in the course of the growth of the ambulacra of the regular Echinoids have for many years been utilized for purposes of classification of those forms, but the similar features which exist among some of the irregular types have not been studied in such detail as to render them systematically useful. It is with a view to showing that the structure of the ambulacra in one group of the Echinoidea Irregularia can be relied upon as an index of the relationships of the various genera, that this note has been written. It makes no pretence at completeness, as the material and time available have not been sufficient to allow of an exhaustive study. But the results of those observations that have been made seem to indicate that among the Holectypoida it should be possible, with the aid of zonal collecting, to trace genetic series among the various species, and perhaps to bridge the gulfs existing between the genera, by a study of their ambulacral plating.