ORDOVICIAN BRACHIOPODA 35 



Betton, Meadowtown and Rorrington Beds. The Meadowtown assemblage is 

 topotypic with those specimens on which J. de C. Sowerby (in Murchison 1839 : 

 pi. xxii, fig. 13) based P. attenuata. Comparisons of internal features like the relative 

 anterior extension of the pseudointerareas and the dorsal median ridge, as well as 

 the depth and outline of the valves (Tables 15-18), show that those Palaeoglossa 

 recovered from the Betton and Rorrington Beds are indistinguishable from P. 

 attenuata. 



The small sample taken from the Mytton Flags, on the other hand, differs from the 

 others in the significantly greater expansion in width relative to length during shell 

 growth (Table 15). The beak of the brachial valve is also more acutely rounded and 

 the lenticular pseudointerarea correspondingly more restricted to the umbonal 

 region, while the sporadically distributed shallow pits indenting the postero-medial 

 internal surfaces of the Mytton shells have not been seen in younger specimens. 

 These differences merit specific recognition ; and it is interesting to note that the new 

 species appears to be restricted in stratigraphic distribution because the few specimens 

 of Palaeoglossa known from the Weston, Spy Wood and Whittery Formations are 

 best assigned to attenuata. 



Subfamily GLOSSELLINAE Cooper 1956 

 PSEUDOLINGULA Mickwitz 1909 emended A. W. 



1909 Pseudolingula Mickwitz : 771. 



1945 Pseudolingula Mickwitz ; Sinclair : 58. 



1965 Pseudolingula Mickwitz ; Rowell in Williams et al. : H267. 



1969 Pseudolingula Mickwitz ; Goryanski : 41. 



Diagnosis. Biconvex, parallel-sided glossellinids ornamented by fila with an 

 orthocline ventral pseudointerarea divided by a pedicle groove into two striated 

 propareas and an undifferentiated dorsal posterior margin ; umbonal muscle scars 

 divided, other muscle bases, including identifiable lateral and transmedian sets, 

 supported on a broad adnate platform in the pedicle valve and impressed on either 

 side of a median septum in the brachial valve ; interiors sporadically pitted postero- 

 medially. 



Discussion. Well-preserved internal moulds of Pseudolingula recovered from 

 the Ordovician rocks of the Shelve area reveal more details of the internal mor- 

 phology of this genus than have hitherto been recorded, and allow for a reappraisal 

 of its position within the lingulide hierarchy. All characters confirm the lingulacean 

 affinities of the taxon, and the divided aspect of the umbonal muscle bases indicates 

 that it is best assigned to the Obolidae. This allocation is corroborated by the 

 development of a pedicle groove bounded by flexures and striated propareas at least 

 in the specimens from the Rorrington Beds. The undifferentiated dorsal posterior 

 margin, however, suggests that Pseudolingula is closer to the Glossellinae than the 

 Lingulellinae as currently conceived (see Rowell in Williams et al. 1965 : H266-269), 

 and its transference to the former subfamily is accordingly adopted for this study. 



