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74 Ajjierican Seash ells
100,000 or more living, and many more fossil, species of moUusks. It is im-
possible to avoid using technical names for various parts of the shell and its
animal, such as apex, spire, whorls, operculum, etc., for most of these words
have no counterpart in everyday language. Familiarization with these few
terms is gained easily and rapidly as trial identifications and references to the
illustrated glossaries are made. Many of the technical terms explained below
are not employed in this book, but they are presented for the sake of those
readers who intend to use more advanced works.
Gastropod Features
Shape of shell. It is this character that is instinctively used at first when
identifying a snail shell, and little would be gained in discussing at length
what our photographs so clearly demonstrate. However, the shape of the
adult shell in some species may differ radically from its young stages as may
be seen in the illustrations of the cowries (pi. 6g) or the American Pelican
Foot (Aporj'hais, pi. 23c). Monstrosities caused by embryological defaults
or by injury in early life have always been a source of error in identification,
and in certain extreme cases many species have been erroneously described
as new.
Parts of the shell. As the typical gastropod mollusk grows, it adds to
the spiral shell and produces turns or ivhorls. The first few whorls, or nu-
clear whorls, are generally formed in the tgg of the mollusk and usually
differ in texture, color and sculpturing from the postnuclear whorls which
are formed after the animal has hatched. When the nuclear whorls are
marked off from the remainder of the whorls they are often referred to as
the protoconch. The last and largest whorl which terminates at the aperture
of the shell is known as the body ivhorl. The periphery is an imaginary
spiral area on the outside of the whorl, usually halfway between the suture
and the base or at a point where the whorl has its greatest width. The Giant
Atlantic Pyram (pi. 4q) shows a narrow color band on the periphery of the
last whorl. The w^horl just before the last whorl often has distinctive characters
and has been differentiated by the name penultimate njohorl. Above
this the succeedingly smaller earlier whorls in the pointed apex of the shell
are known as apical ^whorls. The rate of expansion of the growing whorls
and the degree to which the succeeding whorls "drop" determine the shape
of the shell. The sides of the whorls may be fiat, globose, concave, chan-
neled or ribbed. The juncture of each whorl against the other forms a suture
at the top or above the shoulder of each whorl. The suture may be very
fine—a mere tiny, spiral line—or it may be deeply channeled (see Busy con
canaliculata, the Channeled Whelk, pi. 2 3n). Sutures may be wavy, irreg-
ular, slightly or deeply indented or impressed.