Protobranchia

Protobranchia is a subclass of bivalve molluscs. It contains the extant orders Nuculanida, Nuculida, and Solemyida.

These are deep water clams of a small and primitive order with a taxodont hinge (composed of many similar, small teeth), generally with a central ligament pit, large labial palps which are used in deposit feeding and the gills used only for respiration, the anterior and posterior adductor muscles are nearly equal in size, a foot which is divided sagittally and longitudinally with papillate margins. The foot in Protobranchia clams is without a true byssus gland, although they frequently have a nonhomologous byssal gland in the heel. The byssal gland in these clams does not produce threads as in Pteriomorphia.

2010 Taxonomy
In 2010 a new proposed classification system for the Bivalvia was published in by Bieler, Carter & Coan revising the classification of the Bivalvia, including the subclass Protobranchia.

Subclass: Protobranchia

Order: Nuculanida

 * Superfamily: Nuculanoidea
 * Family: Bathyspinulidae
 * Family: Lametilidae accepted as Phaseolidae
 * Family: Malletiidae
 * Family: Neilonellidae
 * Family: Nuculanidae
 * Family: Siliculidae
 * Family: Tindariidae
 * Family: Yoldiidae

Order: Nuculida

 * Superfamily: Nuculoidea
 * Family: Nuculidae
 * Family: Sareptidae

Order: Solemyida

 * Superfamily: Manzanelloidea
 * Family: Manzanellidae
 * Family: Nucinellidae
 * Superfamily: Solemyoidea
 * Family: Solemyidae

Order: † Praecardiida

 * Family: † Butovicellidae
 * Genus: † Butovicella
 * Family: † Praecardiidae
 * Genus: † Slava
 * Genus: † Cardiola
 * Family: † Antipleuridae
 * Genus: † Dualina
 * Genus: † Hercynella