ORDOVICIAN BRACHIOPODA 13 



also narrowly ranging in the Llandeilo area where it occurs in ashy shales of the 

 Didymograptus bifidus zone and in the succeeding basal beds of the Ffairfach Grit 

 at Ffairfach. Tissintia immatura, on the other hand, is found throughout the 

 Llandeilo Flags (Williams 1953 : 190-194), while its so-called 'variety' T. immatura 

 plana is also recorded from conglomeratic sandstones (Williams 1953 : 185) recog- 

 nized as the top member of the Ffairfach Group-specimens identified as this sub- 

 species, from the basal Ffairfach Grit at Garn-wen, actually belong to T. prototypa 

 (Williams 1953 : 184). In the Shelve area, T. immatura is restricted to the Meadow- 

 town Beds ; presumably its absence from the Rorrington Beds, which are now 

 correlated with the Middle and Upper Llandeilo Flags (Bassett in Williams et al. 

 1972 : 33), may be attributed to adverse environmental conditions. The distribution 

 of Tissintia in Shropshire and Carmarthenshire therefore suggests that the Betton 

 Beds and at least part of the Weston Beds should be correlated with the Ffairfach 

 Group, and the Meadowtown Beds with the Lower Llandeilo and, possibly, the top 

 conglomeratic member of the Ffairfach Group. 



Other brachiopods tend to confirm this correlation. Sowerbyella antiqua is known 

 from the Betton Beds and from the lower part of the Meadowtown Beds, and although 

 it is common at certain horizons in the Ffairfach Group and Lower Llandeilo, it is 

 unknown from the Middle and Upper Llandeilo Flags. The small Dalmanella 

 found in both areas constitute two morphologically distinctive stocks. But the 

 differences between them may have been phenotypic, and it is noteworthy that they 

 first appeared in the Betton and Ffairfach successions simultaneously with Sower- 

 byella. Oxoplecia nantensis, which was first described by MacGregor (1961 : 196) 

 from the Upper Llandeilo rocks of the Berwyn Dome, is widely distributed in the 

 Llandeilo strata of S.W. Wales and a related form has been recovered from the 

 Ffairfach Group. Horderleyella on the other hand, which is well represented in 

 Welsh successions, rarely occurs, and then only as an indeterminate species in the 

 Meadowtown Beds. Moreover the Shropshire strophomenid has surprisingly 

 proved to be a new species of Rafinesquina unrelated to its well-known contemporary 

 Macrocoelia llandeiloensis. This unexpected discovery suggests that the Welsh 

 Llanvirn-Llandeilo strophomenids require revision because those previously identi- 

 fied as the species 'llandeiloensis' may prove to belong to two different genera. 



The graptolite-bearing Rorrington Beds and the trilobite-rich Middle and Upper 

 Llandeilo Flags have different biofacies. Each has yielded a subordinate but 

 distinctive brachiopod assemblage, with a predominance of inarticulates in the 

 Rorrington Beds and of Tissintia and Dalmanella in the Llandeilo Flags. Indeed 

 articulate brachiopods found in the Rorrington Beds are usually immature specimens 

 indeterminate at infrageneric level, although Bicuspina, Heterorthis and Skenidioides, 

 as precursory elements of the Spy Wood fauna, deserve attention. The first two, 

 together with Gelidorthis cf . partita, are reminiscent of Middle Caradocian assemblages 

 of Morocco (Havlicek 1971) and Czechoslovakia (Havlicek and Vanek 1966 : 54-55). 

 This earlier appearance in the Anglo-Welsh successions than in Bohemian or African 

 strata of articulate brachiopods seems to have been part of a recurrent pattern, 

 possibly representing the effects of diachronic migration . Thus Tissintia is not known 

 in Morocco until mid-Llandeilo times ; and Tazzarinia, which has been collected from 



