ORDOVICIAN BRACHIOPODA 89 



of the entrance to Yewtree Level, The Hollies, Snailbeach (Grid Ref. S J 380018). 

 The valve was subcircular in outline and about one-sixth as deep as long with evenly 

 convex lateral areas and a strong median sulcus, represented by a broad median 

 ridge on the valve interior, becoming fainter anteriorly. Internal impressions of 

 ribbing indicate that the valve tended to be multicostellate with 5 costellae per mm 

 at the antero-medial margin. The short planar interarea is anacline, divided medially 

 by a notothyrium with a low platform bearing a linear cardinal process, and bounded 

 by short pointed brachiophores with subparallel bases extending forward for one- 

 eighth the length of the valve and sockets defined by fulcral plates. The adductor 

 muscle scar impressions are faint and incomplete. 



In the absence of impressions of the pedicle valve, identification of the specimen 

 is not a certainty. However, the presence of a sulcus and the subparallel attitude 

 of the brachiophore bases favour its assignment to Nocturniella. Yet it is quite 

 distinct from the only described species of that genus, N. nocturna Barrande from 

 the Arenig Komarov Beds of Czechoslovakia (Havlicek 1950 : 125), especially in the 

 plate-like aspect of its cardinal process and the anterior suppression of the sulcus. 



Family DALMANELLIDAE Schuchert 1913 



DALMANELLA Hall & Clarke 1892 



Dalmanella parva Williams emended 



(PL 15, figs. 1-4, 7) 



1949 Dalmanella parva Williams : 169. 



Diagnosis. Small, ventribiconvex Dalmanella with a brachial valve 77% as 

 long as wide, and with fine costellae, rarely branching externally, numbering about 

 6 per mm at the 2 mm growth stage ; dental plates 23% as long as the pedicle valve 

 and 130% as divergent as long ; subparallel brachiophore bases 22% as long as the 

 brachial valve and separated from each other by 97% of their length. 



Description. Small, ventribiconvex Dalmanella with obtuse cardinal angles ; 

 pedicle valve 32% as deep as long with a carinate rounded median zone and evenly 

 sloping lateral areas ; brachial valve 77% as long as wide and about one-fifth as deep 

 as long with a shallow median sulcus almost three-quarters as wide as valve length 

 flanked by evenly convex lateral areas ; ventral interarea slightly curved apsacline 

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

 callist usually conspicuous ; radial ornamentation costellate commonly 6 per mm, 

 2 mm antero-medially of the dorsal umbo ; branching simple, rarely developing 

 secondaries especially externally (only 3 external costellae observed in sectors III 

 and IV of 24 well-preserved exteriors). 



Teeth small, supported by dental plates extending anteriorly for 23% the length 

 of the pedicle valve and diverging for 130% their length ; ventral muscle field 

 bilobed with diductor scars extending anteriorly for 35% the length of the pedicle 

 valve but not surrounding the median adductor field ; vascula media slightly diver- 

 gent, remaining mantle canal system obscure. 



