ORDOVICIAN BRACHIOPODA 



147 



cardinal process 



-tooth socket 



\ . ridge 



•O* — P lu 9 



muscle scar 



A B 



Fig. 11. Diagrammatic views of (A) the ventral and (B) the dorsal interiors of 



Bystromena, 



and 8 ribs per mm, 5 mm antero-medially of the umbones, of 3, i, 4 and 1 valves 

 respectively ; ventral and dorsal interareas apsacline and anacline respectively with 

 a functional foramen at least in young pedicle valves, and well-developed pseudo- 

 deltidium and chilidium. 



Teeth obliquely disposed ridges supported by widely divergent dental plates 

 extending anteriorly for less than one-tenth of the pedicle valve ; ventral muscle 

 field obscure, cylindroid infilled pedicle tube, with a shallow median furrow on the 

 dorsal surface, extending anteriorly for about 1 mm ; floor of mature valve orna- 

 mented by many radiating thin partitions. 



Cardinal process small with vestigial median ridge present between the lobes in 

 immature valves, lobes continuous with widely divergent socket ridges subtending 

 an angle of about 135 ° ; notothyrial platform absent except in mature valves when 

 it extended forward for a short distance as a median ridge bifurcating anteriorly ; 

 transmuscle septa present but usually obscured by radiating thin partitions delineat- 

 ing a pinnate mantle canal system ; dorsal muscle impressions obscure. 



Type material. 



Holotype Internal mould of pedicle valve (BB 35363) 

 Paratypes External and internal moulds of incomplete 



pedicle valve (BB 35364a, b) 



External and internal moulds of pedicle valve 



(BB 35365a, b) 



External and internal moulds of pedicle valve 



(BB 35366a, b) 



External and internal moulds of brachial valve 



(BB 35367a, b) 



External and internal moulds of brachial valve 



(BB 35368a, b) 



Incomplete internal mould of pedicle valve 



(BB 35369) 



Incomplete internal mould of brachial valve 



(BB 35370) 



length 

 8-0 



6-o 



width (mm) 

 "•5 



8-o 



4-5 



5'0 



4-0 



6-5 



