ORDOVICIAN BRACHIOPODA 131 



exclusively parvicostellate with counts of 9, 10, 11 and 12 ribs per mm, 2 mm antero- 

 medially of the umbones of 6, 30, 19 and 9 brachial valves and differentiated into 

 sectors with a mean width of 0-57 mm, 2 mm antero-medially of the umbo in 24 out 

 of 64 valves ; rugae with a wavelength of 0-15 mm rarely developed sporadically in 

 postero-lateral areas. 



Teeth small, dental plates obsolescent ; ventral muscle field strongly cordate in 

 outline, 74% as long as wide and extending forward for 32% of the length of the 

 pedicle valve ; adductor scars small, impressed as two hollows in the secondary 

 shell of the umbo and divided by a fine median ridge ; diductor scars splayed 

 anteriorly and divided by divergent vascula media of a lemniscate mantle canal 

 system. 



Cardinalia consisting of a median cardinal process fused with chilidial plates and 

 widely divergent socket plates extending anteriorly for 36% of their lateral spread 

 and for 12% of the length of the brachial valve ; notothyrial platform poorly devel- 

 oped ; lophophore and muscle supports consisting of a median septum almost 

 invariably flanked by 3 pairs of lateral septa and extending anteriorly for 56% of the 

 length of the brachial valve, commonly ankylosed to form an elevated cleft platform 

 in adult valves more than 4 mm long. 



Figured material. 



External mould of brachial valve (BB 35524) 

 Incomplete internal mould of brachial valve (BB 35525) 

 External mould of brachial valve (BB 35526) 

 Internal mould of pedicle valve (BB 35527) 

 External mould of pedicle valve (BB 35528) 

 Incomplete internal mould of brachial valve (BB 35529) 

 Internal mould of brachial valve (BB 35530) 

 Incomplete internal mould of brachial valve (BB 35531) 

 Internal mould of brachial valve (BB 35532) 

 Internal mould of pedicle valve (BB 35533) 

 Incomplete internal mould of pedicle valve (BB 35534) 



Horizon and locality. Flags of the Ffairfach Group exposed on the western 

 side of Coed Duon near Llangadog (Grid Ref. SN 709256). 



Sowerbyella cf. antiqua Jones 

 (PL 23, figs. 2, 5-13) 



Diagnosis. Like Sowerbyella antiqua but with a ventral muscle scar extending 

 forward for only 26% of the length of the pedicle valve. 



Description. Semicircular, concavo-convex Sowerbyella with acute cardinal 

 angles, brachial valve 50% as long as wide, pedicle valve 15% as deep as long ; 

 ventral and dorsal interareas apsacline and anacline respectively with a supra- 

 apical foramen and a small pseudodeltidium and complementary chilidium ; orna- 

 mentation exclusively parvicostellate with counts of 8, 9, 10, 11 and 12 ribs per mm, 



length 



width (mm) 



4'5 



9-2 



5-0 



io-o 



6-0 



8-5 



6-o 



- 



3-o 



- 



3'4 



7-0 



4-6 



- 



5-o 



io-o 



5-o 



9-0 



2'5 



- 



