EXERCISE VI. 
THE PERIWINKLE (Diloma). 
Sub-kingdom—Mouuvusca., Class—GASTEROPODA. Order—BRANCHIATA. 
1. There are two different species of Diloma that go 
under the name of Periwinkle. One called D. cethiops 
has the mouth of the shell surrounded by a black, 
or blue-black, band ; the other, D. wndulosa, has a 
bright yellow margin outside the black band. UD. 
cethiops is common all over New Zealand; D. wn- 
dulosa is common in the south, 
2. External characters.—The shell is regularly spiral, 
and dextral; that is, the spiral is twisted in the 
direction that the hands of a watch turn. The con- 
volutions of the shell are called whorls, and the solid 
axis around which they wind is the columella. The 
line formed by the junction of two whorls is called a 
suture. The last turn of the shell is the body-whorl, 
the others form the spire. The aperture of the shell 
is entire, and the anterior end of the columella is 
produced into a ridge round the inner margin of the 
outer lip of the aperture. 
8. Place the animal in a saucer of salt water. When it 
comes out, examine the animal. Observe the 
elongated muscular foot by which it moves; at the 
posterior upper end is the round horny operculwm. 
The head has a short truncated proboscis which 1s 
transversely striated; on either side is a long 
