1890.] Zoölogy. 479 
[based on examination of examples of each which I submitted to you] 
which are Echinometra vanbrunti A. Ag., and Cidaris thouarsii 
Val. These were all collected on the sides of a great open sea-cave in 
the face of a beetling cliff in the outer portion of Guaymas Harbor, 
„and were taken without exception, so far as I now remember, from 
holes in the solid rock. This was my first knowledge of the rock- 
boring habit of sea-urchins ; but as I supposed the habit to be general 
with these species, and the matters of my observation to be well-known 
to all other naturalists, I did not venture to publish it. One fact which 
I observed, however, seems to me to be at variance with one of your 
conclusions. In collecting the urchins, I soon learned that if I wished 
to extract the specimens without injury to themselves, or, in case of the 
Echinometra, to avoid total failure, made more pointed by pricking my* 
fingers, I must make up my mind in advance just how to seize them to 
best advantage and then to withdraw them quickly. If I was awkward 
or slow in any case, or hesitated and took a fresh hold from pricking 
my fingers, the Echinoderms took occasion to ‘‘set’’ a multitude of 
spines against the walls of their ‘‘ geode,” and, thus braced, could 
usually defy further efforts to remove them, save by such harsh means 
as fractured the bodies of the animals. I am therefore constrained to 
believe that on being disturbed by the violence of tide and storm- 
waves, to which they are here greatly exposed, they would set the spines, 
as they do in a case of human interference, It would certainly then 
be a rare thing that the action of the water should move them, though 
it is obvious that if it did succeed in moving one or more of its spines 
a little, the mutual erosion of spine and rock would be greater than if 
the spines were not braced. It seems to me, therefore, that very little 
erosion, so far as the action of the spines is concerned, would be caused 
by the sea-urchin washing about in the geode, but that the wear pro- 
duced by the spines, and on the spines themselves, would be due to 
what might be called the walking of the spines; that is, the removing 
of aspine here and there from its position to such new positions as 
circumstances demanded. That certain of the spines are so used and 
become somewhat worn in the case of the two species above men- 
tioned, is possibly indicated by the fact that some of them are obliquely 
truncated ; yet the truncated end seems to be hardly smooth enough for 
a worn surface, and is perhaps asurface of fracture. Many of the spines 
have the tip perfect, but it is noticeable that in the Cidaris, whose 
spines are usually incrusted with species of Serpula and Spirorbis, the 
tip of the spine is bare or nearly so, and in some instances appears to 
have been worn bare. The rock in which the excavations were made 
