ORDOVICIAN BRACHIOPODA 51 



Dorsal interior with a simple cardinal process, short divergent brachiophores, and 

 suboval adductor scars divided by a low median ridge and extending forward for 

 about half the length of the brachial valve. 



Figured material. 



length width (mm) 

 External moulds of conjoined valves (BB 35497) 5-2 8-0 



Incomplete external and internal moulds of pedicle valve 

 (BB 35498a, b) 5-o 



Incomplete external and internal moulds of brachial valve 

 (BB 35499) 2-0 



External moulds of conjoined valves (BB 35500) - 5-5 



Horizon and localities. Mytton Flags : BB 35498 from crag in the wood of 

 Crowsnest Dingle, 500 yds north of Blakemoorflat (Grid Ref. SJ 374012) ; other 

 specimens from exposures in the adit in Maddox's Coppice, 1200 yds ENE of St 

 Luke's Church, Snailbeach (Grid Ref. SJ 382030). 



Discussion. The few moulds of Orthis s.s. recovered from the Mytton Flags are, 

 when allowance is made for the small size of the shells which were rarely more than 

 5-5 mm long, best identified as Orthis callactis Dalman, the type species for the genus. 

 Illustrations of Russian representatives of the species from strata of B ni age 

 (Schuchert and Cooper 1932 : PI. 2 ; Alichova 1953 : PL 1) show that adult shells 

 were more subquadrate than semi-oval and the cardinal extremities less acute than 

 those of the Mytton specimens. But growth lines indicate that during adult develop- 

 ment there was an acceleration of forward growth relative to lateral increment and 

 that the outline, profile and ornamentation of young shells rendered them in- 

 distinguishable from the Shropshire specimens. 



Orthis sp. 



(PL 8, fig. 1) 



A few dorsal internal moulds from the Stapeley Volcanic Group, exposed 220 yds 

 SSE of Whitsburn Cottage, Leigh (Grid Ref. SJ 332034), are best assigned to Orthis. 

 The larger (BB 35425) which is 4 mm long and about 6-5 mm wide is the mould of a 

 very slightly convex semicircular valve with squared cardinal angles which was 

 ornamented by about 16 rounded costae and bore a linear cardinal process on a low 

 notothyrial platform. The brachiophores, bounding the notothyrium and defining 

 the narrow sockets, were short, widely divergent, and disposed at about the same 

 angle as the almost hypercline interarea. 



The rounded costae, the slight convexity and the squared cardinal extremities 

 suggest that the specimens are not assignable to 0. callactis Dalman. But it seems 

 advisable to withhold systematic recognition until a sample of the pedicle valves 

 has been recovered and more is known about the variability of the brachial 

 valve. 



