854 The American Naturalist. [October, 
trail of mucus in the same way that a land snail (Helix) 
leaves a trace of secretion in his wake. Besides the extra- 
neous growth on these shells, they are the home of numerous 
pholads which burrow into the shell the same as into soft rock. 
The little domiciliary squatters often cause protuberances in 
the interior of the shell where the borer has drilled through 
the epidermis and calcareous portions into the nacre, which is 
always supplied sufficiently to resent the encroachments of 
domiciliaries. Dr. Robert E. C. Stearns of the National 
Museum has written an interesting paper on animals that 
encroach. on the domain of others,’ and it is illustrated with a 
plate showing these protuberances in an abalone shell. A red 
abalone that showed, on the inside, the raised nodule or pro- 
tuberance indicative of the presence of a small rock-borer, on 
the outside of the abalone showed no perforation as usual, but, 
instead, there was a round depression of nacre, the pholad 
(? Penitella parva Tryon) had been completely covered with nacre, 
but a hammer and a chisel discovered the little bivalve that 
had been sealed up in his own domicile. AsI broke the little 
pholad in getting it out of the abalone shell it could not be 
identified otherwise than doubtfully. 
As pearls consist of coatings of nacreous secretion they are 
sometimes found in abalone shells. "These will not compare 
with pearls found in the pearl oyster, as the latter areunrival- 
led. Pearls in abalones are often pear-shaped and green in 
color, in fact some of these so-called “pearls” are peculiar 
rather than beautiful. One fine pearl baroque (irregular) was 
taken from under the columella margin of a green abalone. 
It is the property of Mrs. Prof. Lowe of Pasadena, S. Califor- 
nia, and is about 2? inches long; it is three-cornered in shape, 
and atthe widest and thickest part it is 2} inches around. 
As is well known the habitat of abalone is among rocks, 
where, at very low tide, they may be found huddled together 
in a corner of a rock in a rock pool, or hedged in between fis- 
sures of immense rocks, always as though hiding from the 
rtain Parasites, Commensáls and Domicilíaries in the Pearl Oyster, etc. 
eed € 1886, pages 339-344, with three plates). 
PIG MT asit Ar ee 
