Formerly, one of the two classes of the phylum Brachiopoda. The phylum comprises solitary, exclusively marine, coelomate (having a true coelom or mesodermally lined body cavity), lophophorate (having a lophophore, a food-gathering ciliated organ), bivalved animals, with the two valves symmetrical about a median longitudinal plane. Now the former Inarticulata comprises two of the three subphyla currently recognized, Linguliformea and Craniiformea, together with a few taxa considered to be basal rhynchonelliform brachiopods. These two subphyla encompass approximately 6% of the generic diversity of the Recent (Holocene, from 10,000 years ago to the present day) and fossil brachiopods.

The name Inarticulata refers to the lack of tooth-and-socket articulatory structures on the two valves of these brachiopods. The name was abandoned because of recent changes made in the classification thought to reflect a better understanding of phylogenetic relationships among several groups of Cambrian brachiopods. Because some linguliform and craniiform species have primitive forms of articulation that may or may not be homologous with articulated brachiopods, and because some early rhynchonelliforms lack articulation, the term Inarticulata no longer accurately characterizes this group of brachiopods. However, molecular and morphological evidence suggests that Linguliformea and Craniiformea are each likely to be monophyletic (evolved from a single interbreeding population), are sister groups to one another, and may form a clade, together with Phoronida at the base, that is the sister group to Rhynchonelliformea.