A REVIEW OF THE MORPHOLOGICAL FEATURES AFFECTING THE CLASSIFICATION OF CLITAMBONITIDINE BRACHIOPODS

It is difficult to provide a monothetic diagnosis of a clitambonitidin e brachiopod, as any important feature (chilidium, deltidium, spondyli um, etc.) of a 'standard' genus such as Vellamo may be absent from ano ther stock that is nevertheless a quite acceptable member of thee subo rder. Critical examination is made of the morphological structures, pa rticularly those associated with the vital functions of maintaining po sition relative to the substrate and of feeding. The pedicle foramen b ecame sealed in numerous stocks; a sequence of stages from an open del thyrium via deltidial plates to a complete plate confirms a deltidium rather than a pseudodeltidium; the chilidium likewise varies from a la rge structure, the outer growing surface of which lies within the conc h, to a more modest normal arrangement, a pair of chilidial plates or an open notothyrium. The spondylium supporting the ventral muscle fiel d is variously modified or may only be simulated by a pseudospondylium ; the dorsal muscle field is disported in various ways and, with other variable structures normally accorded taxonomic status like the ancil lary cardinal process ridges and the presence of aditicules, illustrat es the fundamental taxonomic tenet of features unstable in one group b ecoming stable and of taxonomic importance in another.