. Bulletin of the British Museum (Natural History), Geology. PRODUCTELLID AND PLICATIFERID BRACHIOPODS 103 spines rare anteromedially. Short lateral ridges in both valves. Discussion. The genus was placed in synonymy with Pro- ductina (Brunton, 1966) because the dorsal valve interiors of Productina sampsoni, figured by Muir-Wood & Cooper (1960: pi. 123, fig. 9) resembled extremely closely the dorsal interior of A. margahtaceus. The figure in Muir-Wood & Cooper (1960: pi. 123, fig. 17) purporting to be of A. margaritaceus is not of this species nor, we think, the genus; it may well belo
. Bulletin of the British Museum (Natural History), Geology. PRODUCTELLID AND PLICATIFERID BRACHIOPODS 103 spines rare anteromedially. Short lateral ridges in both valves. Discussion. The genus was placed in synonymy with Pro- ductina (Brunton, 1966) because the dorsal valve interiors of Productina sampsoni, figured by Muir-Wood & Cooper (1960: pi. 123, fig. 9) resembled extremely closely the dorsal interior of A. margahtaceus. The figure in Muir-Wood & Cooper (1960: pi. 123, fig. 17) purporting to be of A. margaritaceus is not of this species nor, we think, the genus; it may well belong to the linoproductids. The proliferation of more recently described species within the subfamily allows discrimination to be made between the two genera. Argen- tiproductus is commonly 20 to 25 mm wide across the body cavity ( excluding ears). At a distance of 10 mm from the umbo about 6 to 10 ribs occur in 5 mm width; they are closely spaced, with flat to gently rounded crests. Because the ribs widen anteriorly and branch rarely, the number of ribs per unit width is variable. However, the ribbing on Productina species is finer, with rounded crests, and is more even over the entire shell. Productina is less wide but has a more strongly inflated ventral umbo giving steep flanks and a more strongly convex lateral profile than Argentiproductus. Spines are confined to ventral valves, but those on Argentiproductus are more prolific. Juvenile clasping spines (see Brunton, 1966) normally formed a ring just anterior to the initial point of pedicle attachment, the pedicle sheath, at about 0-5 mm from the tip of the umbo (Figs 13a, b). Thereafter a line of increasingly large support spines grew towards the anterior edges of the weakly defined ears, to a total of about six on each side. In addition the type species, and others in the genus, have one to three spines close to the mid-line, commonly only posteriorly, but rarely anteriorly also. Spines ji in Productina are confined to
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