ORDOVICIAN BRACHIOPODA 65 



Small trigonal teeth supported by narrowly divergent dental plates extending 

 forward for 20% of the length of the pedicle valve ; ventral muscle scar normally 

 elongately pentagonal, with relatively wide median adductor scars flanked by some- 

 what shorter diductor scars, wider than long in young valves but becoming much 

 more elongate in adult shells with a mean width of 97% of the length and extending 

 anteriorly for 38% of the length of the valve. 



Cardinal process simple and blade-like, brachiophores divergent and pointed with 

 the secondary shell, forming the boundaries of the denticulate sockets and noto- 

 thyrium, elevated above the brachial valve floor to simulate fulcral plates ; convergent 

 brachiophore bases extending forward for 19% of the valve length and 48% as long 

 as wide ; adductor muscle field quadripartite with a pair of suboval scars impressed 

 on either side of the median ridge and extending anteriorly of the umbo for 52% of 

 the length of the valve. 



Type material. 



length width (mm) 



Holotype External and internal moulds of brachial valve 



(BB 35505a, b) 47 6-0 



Paratypes External and internal moulds of pedicle valve 



(BB 355o6a, b) 4-5 5-5 



Internal mould of pedicle valve (BB 35507) 5-0 6-5 



Internal and incomplete external moulds of 

 brachial valve (BB 35508a, b) 5-5 7-5 



Internal mould of pedicle valve (BB 35509) 6-5 7-0 



Type horizon and locality. All specimens from exposures of Spy Wood Grit, 

 1100 yds NNE of Rorrington (Grid Ref. SJ 303015). 



Discussion. Remains of Glyptorthis have been recovered from the Meadowtown 

 Beds, Whittery Shales and Spy Wood Grit, but are common only in the last-named 

 formation which has furnished the sample used in establishing a new species most 

 closely related to G. nantensis McGregor (1961 : 187) from the Upper Llandeilo of 

 the Berwyn Hills. However, even in the absence of precise information about the 

 variability of the Welsh species, its pedicle valve is obviously not only much deeper 

 but is further characterized by an interarea which is 'slightly apsacline, almost 

 orthocline', and a significantly larger muscle scar. 



Other penecontemporaneous Ordovician species from Ireland, Scotland and N. 

 America also differ greatly. According to data kindly provided by Mrs H. Carlisle, 

 in the brachial valve of G. crispa M'Coy from the Tramore Limestone of Ireland the 

 cardinalia were relatively wider while the ventral muscle scar extended anteriorly 

 more slowly during growth than in the Shropshire species. G. balclatchiensis 

 (Davidson) from the Ardmillan Series of Scotland (Williams 1962 : 109), and G. 

 assimilis Cooper (1956 : 361) from the Lower Ridley of Tennessee, both differ in 

 being significantly wider. The former species also differs in the replacement of the 

 dorsal sulcus by a narrow rounded fold during adult stages of growth, and the latter 

 in being more coarsely imbricate. 



