Clistenterata

Clistenterata, one of the two subdivisions of the class Brachiopoda. They include those forms in which, with rare exceptions, the two valves of the shell are connected by a hinge (hence also known as the Articulata): the shell is composed of long prisms of carbonate of lime obliquely arranged: they have no anus (whence the name "closed guts"). These are the three characters that separate them from the Tretenterata. The Clistenterata include the great majority of the Brachiopoda, and many of the best known forms; for example the common "Lamp Shell" (Terebratula), found rarely in the deeper waters of our coasts; also Waldheimia, an allied genus, and Rhynchonella, species of both of which occur living in the British area. Most of the species and several of the families are, however, extinct; thus the Spiriferidae, which had a complicated spiral support for the arms, and the large thick-valved, and often concavo-convex Productidae, are almost limited to the Palaeozoic age. In many extinct species and one living one (Rhynchonella doederleini from Japan) the shells are covered with long spines.