no SHELVE DISTRICT 



median zone and gently convex lateral areas ; brachial valve 79% as long as wide, 

 gently convex or more commonly planar with indistinct sulcus ; ventral interarea 

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

 open ; radial ornamentation fasciocostellate commonly 3 per mm, 5 mm antero- 

 medially of the dorsal umbo, external branches poorly developed in the first three 

 sectors, costellae reflexed postero-laterally, follicular embayments rarely indenting 

 internal surfaces of hinge-line. 



Teeth small, supported by dental plates extending anteriorly for 17% of the 

 length of the pedicle valve and extending laterally for 150% of their length ; ventral 

 muscle field elongately oval extending forwards for 38% of the length of the valve 

 with splayed diductor scars almost enclosing a pair of small suboval adductor 

 impressions separated by a low ridge ; adjustor scar located on dental plates. 



Cardinal process bilobed, differentiated in adult valves into elongate shaft and 

 expanded myophore, notothyrial platform well developed passing anteriorly into 

 low ridge ; brachiophores triangular in outline with divergent bases extending 

 forward for 17% of the length of the valve, becoming massive in adult valves so that 

 they are, on average, only 84% as long as their lateral spread ; oblique sockets not 

 bounded laterally by fulcral plates ; adductor scars quadripartite extending forward 

 for half the length of the valve. 



Figured material. 



External and internal moulds of pedicle valve (BB 35474a, b) 

 External and internal moulds of brachial valve (BB 35475a, b) 

 External and internal moulds of pedicle valve (BB 35476a, b) 

 Incomplete external and internal moulds of brachial valve 

 (BB 35477a, b) 



External and internal moulds of brachial valve (BB 35478a, b) 

 Internal mould of pedicle valve (BB 35479) 



Horizon and localities. Meadowtown Beds : BB 35474, 35475 from loose 

 blocks on rough ground 100 yds south-east of Minicop Farm (Grid Ref. SJ 315018) ; 

 BB 35478 from Bed B in measured section in Meadowtown Quarry (Grid Ref. 

 SJ 312012) ; BB 35479 from Quinton's Quarry in the field 200 yds north-east of 

 Meadowtown Chapel (Grid Ref. SJ 312013). BB 35476, 35477 from exposures in a 

 ploughed field along the strike of Betton Beds occurring 180 yds south-west of a well 

 in the lane-side from Meadowtown to Castle Ring (Grid Ref. SJ 310009). 



Discussion. Tissintia prototypa was first described from the upper part of the 

 Didymograptus bifidus shales of the Llandeilo district (Williams 1953 : 180) and is 

 now known from W. Wales and Builth as well as Shropshire, where it occurs abun- 

 dantly in the Llanvirn Weston Beds. A good sample from the Weston Beds has been 

 statistically compared with a topotypic assemblage. The comparison shows 

 (Tables 77-85) that there is no difference between the two samples in parameters of 

 the shape of the shell, the disposition and size of internal features, or in the radial 

 ornamentation except for a marginally significant delay in the differentiation of the 



mgth 



width (mm) 



8-o 



- 



4-0 



5-o 



9-0 



12-0 



14-0 



- 



10-0 



12-0 



n-5 



- 



