74 Ajjierican Se ash 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 char- 

 acters 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. 



