ORDOVICIAN BRACHIOPODA 99 



Description. Small, ventribiconvex Onniella with obtusely rounded cardinal 

 angles ; pedicle valve 23% as deep as long with a subcarinate rounded median zone 

 and evenly sloping to slightly concave lateral areas, brachial valve 76% as wide and 

 11% as deep as long with a shallow narrow to weak sulcus ; ventral interarea planar 

 apsacline, longer than anacline dorsal interarea, delthyrium and notothyrium open, 

 pedicle callist usually well developed ; radial ornamentation of costellae commonly 

 4 per mm, 5 mm antero-medially of dorsal umbo, external costellae poorly developed 

 in sectors III and IV. 



Teeth small trigonal, supported by dental plates extending anteriorly for 16% of 

 the length of the pedicle valve and diverging for 49% of their length ; ventral muscle 

 field bilobed with the diductor scars extending anteriorly for 29% of the length of 

 the pedicle valve and flanking a shorter submedial adductor scar ; vascula media 

 proximally divergent, remaining mantle canal system unknown. 



Cardinal process consisting of linear shaft and rounded myophore in early growth 

 stages but usually massive and medially cleft in valves more than 3 mm long ; 

 brachiophores short, acutely divergent, with bases, which may be broad and in- 

 dented by a pit, extending forward for 19% of the length of the brachial valve and 

 splaying laterally for 58% of their length ; sockets well defined but without fulcral 

 plates ; adductor scars suboval, impressed on either side of median ridge and 

 extending anteriorly for 57% of the length of the valve ; mantle canal system 

 unknown. 



Type material. 



length width (mm) 

 Holotype External and internal moulds of brachial valve 



(BB 35456a, b) 4-3 5-0 



Paratypes External and internal moulds of pedicle valve 



(BB 35455a, b) 4-8 6-o 



External and internal moulds of brachial valve 

 (BB 35457a, b) 3-0 4-0 



Internal mould of brachial valve (BB 35458) 4-0 



Type horizon and locality. Aldress Shales exposed in the bank of Ox Wood 

 Dingle at the south-west corner of Ox Wood, just north of the Rorrington-Wotherton 

 road (Grid Ref. SJ 290007). 



Discussion. Except for the complementary moulds of a brachial valve 

 (BB 35572a, b) collected from the Spy Wood Grit (PI. 16, figs. 15, 18), Onniella is 

 restricted in the Shelve area to the Aldress and Whittery Shales. Although it is not 

 very common in either formation, two small samples were available for study. They 

 proved to be alike in every respect (Tables 64-72) and obviously represent the same 

 species. They are however significantly different from penecontemporaneous 

 Onniella, such as 0. soudleyensis (Bancroft 1945 : 210) and 0. ostentata (Williams 

 1963 : 405). In comparison with the former species, the Shelve Onniella differs in 

 the more elongate growth of the brachial valve, the relative length and narrowness 

 of the cardinalia and in the early development of a massive bilobed cardinal pro- 

 cess. In respect of the last-named feature the Shelve Onniella is nearer 0. ostentata 



