Phylum MOLLUSCA. The diversity of mollusks is not very great in freshwaters, but increases in brackish and estuarine waters, and is tremendous in marine waters. The major molluscan subgroups found in fresh and brackish waters are snails (Class Gastropoda) and clams and mussels (Class Pelecypoda or Bivalvia). Most species are aquatic, but some are intertidal and terrestrial gastropods are common. A listing of the mollusks found in Hawaiian fresh and brackish waters is provided in CPIE.

	33a	(14) & (16)	External shell consisting of two parts (valves) connected at one end by a hinge. { Animal free and usually buried; OR one valve may be cemented to hard substratum; OR animal attached to substratum by a byssus (grouping of tough threads) ~ Class BIVALVIA	[34]
	33b		External shell not a pair of hinged valves; shell cap-like or coiled, bulbous or spired. { Animal not attached to substratum ~ Class GASTROPODA	[40]
			Key to Bivalve Mollusks
	34a	(33)	Lower valve cemented onto hard substratum or other shells. Marine and estuarine only. Oysters ~ Family OSTREIDAE	[38]
	34b		Animal either not attached (may be buried) or if attached to substratum, then attached by byssal threads	[35]
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	35a	(34)	Attached to hard objects by byssal threads. Mussels	[36]
	35b		Not attached at all, but living buried in soft substrata (mud, sand, or gravel). Clams	[37]
			~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	36a	(35)	Valves inflated, slipper-shaped and with numerous fine, radial ribs and concentric threads; color yellow to dark brown. { 10 to 30 mm long; usually largest where some freshwater enters ocean. Abundant on limestone and sometimes basalt shorelines around 0 tide elevation ~ Family MYTILIDAE Brachidontes crebristriatus (Conrad)
	36b		Valves compressed, rhomboidal, smooth with concentric lamellae (thin plates); color dark purple-brown. { To 41 mm. In dense beds at tide line in brackish water or on marine shore influenced by freshwater. Toothed pearl shell ~ Family ISOGNOMONIDAE Isognomon californicum (Conrad)
			~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	37a	(35)	Ligament (leathery connection at hinge) external. Shell white and ribbed, mostly covered by a thin black to yellow-brown, organic layer (periostracum). To 5 cm. Freshwater streams and estuaries. Asiatic flume clam ~ Family CORBICULIDAE Corbicula fluminea Muller
	37b		Size and color variable, but with internal ligament and without a dark periostracum	[Note B]
			~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	38a	(34)	Valves somewhat irregular, roughly oval, symmetrical; hinge short and muscle scar white. { Native, marine oysters Ostrea spp.	[Note B]
	38b		Valves irregular, elongate, asymmetrical, with attached valve larger; muscle scar colored and eccentric (produced to one side). { Estuarine, in bays and harbors. Introduced oysters Crassostrea spp.
			Japanese oyster -- C. gigas North American bluepoint -- C. virginica

Note B:

Your identification is leading to a group with no fresh or brackish water representatives in the Hawaiian Islands. The identification may be correct for the location where you made the collection or observation (e.g., near the ocean), but the species is marine and may not be covered in the key. If the collection was made in freshwater, you should back up several couplets and confirm your decisions.