"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. Atrypa) or inwards (e.g. Spirifer). These are the only possible alternatives; it is probable that both systems were evolved more than once."