ORDOVICIAN BRACHIOPODA 71 



the fragment of the dorsal exterior available at present, may have included about 

 16 costae, arising just anterior of the umbo. 



Family FINKELNBURGIIDAE Schuchert & Cooper 1931 



DIPARELASMA Ulrich & Cooper 1936 



Diparelasma sp. 



(PI. 11, figs. 3, 7, 8) 



The internal and an incomplete external mould of two brachial valves, 2 and 3 mm 

 long respectively, and an incomplete external mould of a pedicle valve, collected from 

 the Mytton Flags are best assigned to the finkelnburgiid Diparelasma. The moulds of 

 one brachial valve and the pedicle valve (BB 35335a, b) came from a small exposure, 

 20 yds west of a runnel in the valley side north of the col separating Perkin's Beach 

 and Mytton's Beach (Grid Ref. SJ 366002) ; those of the other brachial valve 

 (BB 35336a, b) were obtained from outcrops near the top of the steep tributary to 

 Crowsnest Dingle, 350 yds WNW of Blakemoorflat (Grid Ref. SJ 373008). 



The moulds indicate that the shell was ventribiconvex and transversely sub- 

 quadrate in outline with obtuse cardinal angles, had a sulcate brachial valve about 

 22% as deep as long and 64% as long as wide, and a pedicle valve about one-third 

 as deep as long. Radial ornamentation was finely multicostellate with counts of 7 

 and 9 costellae per mm, 2 mm anterior of the umbones. The cardinalia consisted 

 of blade-like brachiophores diverging from the anacline interarea with bases con- 

 verging onto the floor of the valve on either side of a low median ridge to define a 

 deep notothyrium containing a simple cardinal process. The fulcral plates were 

 more strongly developed in the smaller valve as antero-lateral boundaries to the 

 sockets. A pair of discrete adductor muscle scars extending anteriorly of the umbo 

 for about half the length of the valve were impressed on either side of the median 

 ridge. 



Despite the lack of information about the ventral interior, the distinctive radial 

 ornamentation and cardinalia leave little doubt that the moulds belong to Dipare- 

 lasma. A number of species have been described from the Lower Ordovician, 

 especially the Upper Canadian of N. America (Ulrich & Cooper 1938 : 147-156) ; but 

 the cardinal process and dorsal sulcus are more strongly developed in the Shropshire 

 specimens and may eventually prove to be diagnostic of a new species. 



Family PLECTORTHIDAE Schuchert & Le Vene 1929 



Subfamily PLECTORTHINAE Schuchert & Le Vene 1929 



PLECTORTHIS Hall & Clarke 1892 



Plectorthis whitteryensis sp. nov. 



(PI. 11, figs. 10, 13, 16, 17 ; PI. 12, fig. 1) 



Diagnosis. Semi-elliptical, slightly ventribiconvex, rectimarginate Plectorthis 

 with a pedicle valve 69% as long as wide and 22% as deep as long, ornamented by 



