A review of the morphological features affecting the classification of the clitambonitidine brachiopods

It is difficult to provide a monothetic diagnosis of a clitambonitidine brachiopod, as any important feature (chilidium, deltidium, spondylium, etc.) of a 'standard' genus such as Vellamo may be absent from another stock that is nevertheless a quite acceptable member of the suborder. Critical examination is made of the morphological structures, particularly those associated with the vital functions of maintaining position relative to the substrate and of feeding. The pedicle foramen became sealed in numerous stocks; a sequence of stages from an open delthyrium via deltidial plates to a complete plate confirms a deltidium rather than a pseudodeltidium; the chilidium likewise varies from a large structure, the outer growing surface of which lies within the conch, to a more modest normal arrangement, a pair of chilidial plates or an open notothyrium. The spondylium supporting the ventral muscle field 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 ancillary cardinal process ridges and the presence of aditicules, illustrates the fundamental taxonomic tenet of features unstable in one group becoming stable and of taxonomic importance in another.