852 The American Naturalist. [October, 
to the same treatment shows a black exterior and this is the 
“black abalone.” It is also called the “white abalone” in 
reference to its pearly interior and exterior, if the calcareous 
layers have been ground off leaving only the mother-of-pearl 
on the outside, as is often the case. A species with corruga- 
tions (H. corrugata) presents a reddish-purple color when 
ground off by a skillful workman. All these shells take a 
beautiful polish, but, while the shells are made more attract- 
ive to the popular taste, scientifically their value is depreciated 
after they fall * victims," as Carpenter expresses it, ^to the 
grindstone and acids"  Physicists tells us that the play of 
tints visible in the nacre or mother-of-pearl is caused by the 
action upon light of the tiny layers composing the nacre. 
“These layers are microscopically corrugated and their edges 
meet the rays of light and partly decompose them as do the 
rain drops in a rainbow producing a play of colors" (I once 
dissolved the inner layers of an abalone shell in muriatic acid, 
the dish was placed aside for,several hours and on seeing it 
again I wassurprised to find a beautiful sediment of iridescent 
mother-of-pearl; pressure was applied, and the play of colors 
was gone. The result was new to me at that time and was a 
pleasant surprise). 
Typical shells of the Gastropoda (so named because the 
" under side of the body forms a muscular foot for gliding 
along”), the class to which abalones belong, are spiral in their 
form. Although these shells appear flat,a close inspection 
shows a well developed spire, but in most species, the spire is 
small and the basal or body-whorl is unusually developed and 
depressed, and this gives the shell an appearance as though it 
were only one valve of a bivalve, for which it has often been 
mistaken when seen by persons unacquainted with these 
forms. The shells have a row:of open holes usually from five 
to nine, on one side, but these vary in number as the animal 
grows older; the holes close, until old shells have been seen 
with only one or two holes left open.* These holes are on the 
left side of the shell and through them the tentacles of the 
animal are often protruded. When the animal is resting upon 
*A California Conchologist has a shell with 2// the holes closed. 
