Page:American Seashells (1954).djvu/92

74 100,000 or more living, and many more fossil, species of mollusks. It is impossible 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.

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 (pl. 6g) or the American Pelican Foot (Aporrhais, pl. 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 whorls. The first few whorls, or nuclear whorls, are generally formed in the egg 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 whorl. 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 (pl. 4q) shows a narrow color band on the periphery of the last whorl. The whorl just before the last whorl often has distinctive characters and has been differentiated by the name penultimate whorl. 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 flat, globose, concave, channeled 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 Busycon canaliculata, the Channeled Whelk, pl. 23n). Sutures may be wavy, irregular, slightly or deeply indented or impressed.