|Two views of a left valve of Acila cobboldiae from the early Pleistocene of the Netherlands|
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.
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.
- Superfamily: Nuculanoidea
Order: † Praecardioida
- Family: † Butovicellidae
- Genus: † Butovicella
- Family: † Praecardiidae
- Family: † Antipleuridae
- Protobranchia Pelseneer, 1889. Retrieved through: World Register of Marine Species on 8 July 2010.
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 340. ISBN 0-03-056747-5.
- Bieler, R., Carter, J.G. & Coan, E.V. (2010) Classification of Bivalve families. Pp. 113-133, in: Bouchet, P. & Rocroi, J.P. (2010), Nomenclator of Bivalve Families. Malacologia 52(2): 1-184
- Nuclanoida Carter, D.C. Campbell & M.R. Campbell, 2000. Retrieved through: World Register of Marine Species on 9 October 2010.
- Nuculida Dall, 1889. Retrieved through: World Register of Marine Species on 20 September 2010.
- Solemyoida Dall, 1889. Retrieved through: World Register of Marine Species on 9 July 2010.