io 4 SHELVE DISTRICT 



Horderleyella sp. 



(PI. 17, figs. 2, 3) 



The external and internal moulds of an immature pedicle valve of Horderleyella, 

 2-8 mm long and 3-5 mm wide (BB 35420a, b), have been recovered from the 

 Meadowtown Beds exposed in the lane side 370 yds north-west of Meadowtown 

 Chapel (Grid Ref. SJ 311015). 



The valve was subquadrate in outline with obtuse cardinal angles and about one- 

 quarter as deep as long in the vicinity of a strong narrow median carina although the 

 lateral areas are only very gently convex. The radial ornamentation was strongly 

 fascicostellate with a count of 4 ribs per mm, 2 mm anterior of the umbo. Inter- 

 nally, divergent dental plates, extending anteriorly for almost one-fifth the length of 

 the valve, formed the posterior boundaries of a subpentagonal ventral muscle scar 

 which was about three-quarters as long as wide and extended forward for about 

 three-tenths the valve length. 



The valve cannot be placed unequivocally in any of the described species of Horder- 

 leyella. The relatively fine fascicostellate ornamentation is more diagnostic of the 

 Upper Llanvirn Horderleyella convexa Williams (1949 : 171), for which counts of 3, 

 4 and 5 costellae per mm have been obtained 2 mm anterior of the umbones of 1, 2 

 and 2 brachial valves respectively. The muscle scar, however, is very much wider 

 than even that of H . convexa ; and, although this difference is likely to be attributable 

 to the immaturity of the shell, more material will have to be obtained before specific 

 identification is merited. 



REUSCHELLA Bancroft 1928 



Reuschella horderleyensis Bancroft carinata subsp. nov. 



(PL 17, figs. 4-9) 



Diagnosis. Subquadrate Reuschella with a narrowly carinate and laterally 

 flattened pedicle valve about 23% as deep as long and a deeply sulcate brachial 

 valve 73% as long as wide and 15% as deep as long ; radial ornamentation coarsely 

 fascicostellate with 2 or 3 ribs per mm, 10 mm anteriorly of the dorsal umbo and 

 poorly developed external branching except in the lateral sectors ; ventral muscle 

 scar slightly bilobed and about one-third as long as the pedicle valve ; brachiophore 

 bases extending forward for 14% of the length of the brachial valve. 



Description. Ventribiconvex, subquadrate Reuschella with acute cardinal 

 angles in specimens up to 14 mm long but usually becoming obtuse in mature shells, 

 brachial valve 73% as long as wide and 15% as deep as long with a narrowly rounded 

 sulcus having a mean width relative to the length of 4 valves of 47% (range 39% to 

 55%) '• pedicle valve with a mean depth relative to length of 23% for 3 specimens 

 representing the elevation of a narrow rounded carina flanked by flattened, rarely 

 undulate lateral areas ; ventral and dorsal interareas well developed, apsacline and 

 anacline respectively with open delthyrium and notothyrium ; radial ornamentation 

 coarsely fascicostellate with counts of 2 and 3 ribs per mm, 10 mm anteriorly of the 



