1) Mollusc shell 
A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. The protoconch may sometimes consist of several whorls, but when this is the case, the whorls show no growth lines. The whorls of the adult...
• (n.) The embryonic shell, or first chamber, of ammonites and other cephalopods.
Pro'to·conch noun [ Proto- + conch .] (Zoology) The embryonic shell, or first chamber, of ammonites and other cephalopods.