During a detailed examination of the tests, and in particular of the ambulacra, of the common British species of the Echinoidea Holectypoida, I have found two series of structures the correlation of which, with those known in recent forms, seems difficult. Since I am engaged in the preparation of a description of the British Pygasteridæ, I have thought it advisable to publish now an account of these peculiar features, in the hope that others may be able to throw light on points that seem to me obscure.