The Rostroconchia is a class of extinct mollusks dating from the early Cambrian to the late Permian. They were initially thought to be bivalves, but were later given their own class. They typically have a single, pseudobivalved shell enclosing the mantle and muscular foot. The anterior part of the shell probably pointed downward and had a gape from which the foot could probably emerged. Rostroconchs probably lived a sedentary semi-infaunal lifestyle. The term class, when used by itself, has several meanings in English. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... The Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon_geology and ended about 490 million years BP with the beginning of the Ordovician period. ... The Permian is a geologic period that extends from about 280 to 248 million years before the present (mya). ... Orders Subclass Protobranchia Solemyoida Nuculoida Subclass Pteriomorphia - oysters Arcoida Mytiloida Pterioida Subclass Paleoheterodonta - mussels Trigoinoida Unionoida Subclass Heterodonta - clams, zebra mussels Veneroida Myoida Subclass Anomalosdesmata Pholadomyoida Animals of the Class Bivalvia are known as bivalves because they typically have two-part shells, with both parts being more or less symmetrical. ... A Mantle is a piece of clothing, similar to a robe but open on the front side and often sleeveless. ...