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The lophophores of spire-bearing fossil brachiopods (Spiriferoidea and Atrypoidea) are reconstructed by homological comparison with living brachiopods. It is inferred that all spiral brachidia supported simple spiral lophophores. For each type of brachidium, only one arrangement of the spirolophe could have created an efficient filter-feeding system. The water would have entered the mantle cavity laterally and been ejected medially ; but would have been filtered between the whorls of the conical spiralia either outwards (e.g. Atrypd) or inwards (e.g. Spirifer). These are the only possible alternatives : it is probable that both systems were evolved more than once.

Summary All major types of lophophore in branehiopods, except the ptyeholophe, are represented among the growth stages of the five species described here. The basic ciliary mechanisms are uniform, and similar to those known in other species. The mantle cavity is always divided by the lophophore into inhalant and exhalant chambers with separate apertures; but the arrangement may change radically during ontogeny with the increasing complexity of the lophophore, which is attributed to a dimensional relation between food‐collecting capacity and metabolic requirements. The limited variety of lophophoral arrangements is taken to reflect an inherent limitation imposed by the basic structure and function of lophophores. The feeding mechanisms are compared briefly with those of other filter‐feeding animals.

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