. Bulletin of the British Museum (Natural History), Geology. VISEAN PRODUCTOIDS FROM COUNTY FERMANAGH 205 Discussion. Of the three specimens of Producta plicatilis in the Sowerby Collec- tion only one, () , was figured by Sowerby (1824, pi. 459, fig. 2). This specimen was designated as lectotype by Thomas (1914 : 332) and the Fermanagh shells agree with it in all known respects, such as outline, profile, rugation and ribbing. The type specimen is about one-half the width of the other Sowerby specimens and these have about 28 rugae on each valve (cf. Table 9) but show no sign of
. Bulletin of the British Museum (Natural History), Geology. VISEAN PRODUCTOIDS FROM COUNTY FERMANAGH 205 Discussion. Of the three specimens of Producta plicatilis in the Sowerby Collec- tion only one, () , was figured by Sowerby (1824, pi. 459, fig. 2). This specimen was designated as lectotype by Thomas (1914 : 332) and the Fermanagh shells agree with it in all known respects, such as outline, profile, rugation and ribbing. The type specimen is about one-half the width of the other Sowerby specimens and these have about 28 rugae on each valve (cf. Table 9) but show no sign of ribbing. These two large specimens clearly differ from the type, possibly because of a differing environment, and may not be conspecific. There is a wide variation of size and development of the antero-ventral sulcus in specimens from different localities and it remains to be seen whether the ribbing is associated with other characteristics warranting specific distinction. A comparison of the type specimen of Plicatifera plicatilis with the holotype of P. pseudoplicatilis Muir-Wood reveals that the two are conspecific and it is possible that Muir-Wood's comparison (1928 : 190) was based upon the two large Sowerby specimens. The only differences seen in these type specimens are that P. pseudo- plicatilis has no sulcation and the brachial valve geniculation is slightly less than in P. plicatilis. However, these differences are covered by specific variation. There is no true geniculation in the pedicle valve of P. plicatilis and its even convexity can be seen illustrated by Thomas (1914 : 331, text-fig. 13). Towards its anterior margin the brachial valve is commonly geniculated against the inner surface of the pedicle valve. P. plicatilis is unusual in the depth of the body cavity, which in adult shells is over one-half of the length. The pedicle valve rugation stopped after the development of about 24 rugae, at the lowest, or most ventral part of the valve and immediately poster
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