1890.] Excavations by Sea-Urchins. 15 
décimetre d’eau. C’est dans ces sortes de cuvettes que vivent les 
oursins (Echinus lividus, Lamk.) parqués par groupes de vingt-cinq 
a cent disposés dans un ordre admirable, possédant moins un 
trou; leurs excavations sont tellement rapprochées, qu’il est 
impossible a d’autres oursins de conquérir une place dans la 
colonne. 
“La position des oursins est invariable ; la bouche reste appli- 
quée en bas, au centre du trou; lanus est par conséquent en 
haut. A l’époque de mes observations (août, 1863), les oursins 
gardaient une immobilité compléte,les radioles étant redressées ; 
ils s’engageaient si bien dans les excavations, que souvent leur 
extraction m’en a paru impossible. 
“Je nai pas aperçu d’oursins sur des roches émergées et 
privées d’eau ; et partout où j'en ai trouvé, ils étaient logés dans 
des trous particuliers.” 
From the above quotation, which is but a part of the descrip- 
tion which Dr. Fisher gives of the work of sea-urchins at Biar- 
ritz, it is evident that he carefully studied these excavations at 
this point, but neither from this description nor from the remain- 
der of his account am I able to find that the peculiar relation ob- 
served by Marcou between sea-urchins and pot-holes was seen. 
He makes a minute comparison with the perforations found at 
Biarritz of a number of specimens, which at the time of his writ- 
ing existed in the Museum d’histoire Naturelle at Paris. 
It may not be out of place to call attention to certain pecu- 
liarities in sea-urchin excavations suggested by Prof. Marcou’s 
notes. From the observations it seems that veritable “ pot- 
holes ” were found at Biarritz in which there is a central column 
of rock, which is unlike in its relationship to the pot-hole to the 
columella to the cup of certain corals. This central axis or style 
he supposes to be formed by the wearing away of the rock about 
it, by which it is left standing as a pinnacle.” 
Prof. Marcou believes that the first stage of development of 
the ring-shaped "seis sehen is due to the sea-urchins exca- 
27 It is of know whether thi f th } 
which bounds the 2% pot-holes;” p not a concretionary structure. Prof. Marcou pon 
furnished the d information, for he says that the style is of the same kind of rock as 
that in which the pot-hole is worn. 

