Ro The American Naturalist. [January, 
supposed power of the suckers of the ambulacral feet or the 
other soft structures of the body. Until more facts are advanced 
in support of these theories we can hardly accept them as well 
founded. 
Either the calcareous spines, the wall of the test, or, most 
important of all, the teeth are singly or together the agents 
which have produced these excavations. It seems to the 
author plausible that mechanical movements of the sea-urchin 
by the waves, in combination with voluntary action, has played 
some part in the formation of these cavities. It might facilitate 
our framing a satisfactory answer to the question if we knew 
whether or not the matter worn off from the surface of the sea- 
urchin excavations was always passed into the alimentary canal. 
An examination of the stomachs of sea-urchins which inhabit 
these excavations might by following John’s method throw light 
on this question. Unfortunately I have nothing to add to what 
is already known bearing on this subject. 
An interesting example of erosion and the excavating habit of 
the sea-urchins has been called to my attention by Prof. Jules 
Marcou, of Cambridge. At my request Professor Marcou has 
kindly written out his notes, which have never been published, 
and are consequently quoted entire in the following pages : 
“PERFORATIONS DU CALCAIRE SABLEUX NUMMULITIQUE INFERIEUR 
PAR L’ Echinus lividus DANS LES ROCHES DE HALDE A BIARRITZ. 
“ Entre le vieux Port de Biarritz et la côte des Basques, s’ ad- 
vance la Pointe des Roches de Halde, toujours trés fortement 
battue par les vagues ; a basse marée I’ extrémité de la Pointe se 
découvre sur une longeur moyenne de 75 pieds anglais (25 
metres); et au milieu d’algues diverses on trouve tout une pop- 
ulation littorale (Moules, Patelles, Littorines, Actinies et surtout 
des Oursins). L’oursin est I’espéce commune tout le long de la 
cote de l'Atlantique, Echinus lividus Lam.; non seulement cet 
oursin se creuse une niche dans laquelle il vie mais bien plus il 
agit en corps pour se créer une habitation collective. 
