72 SHELVE DISTRICT 



up to 27 costellae numbering 2 per mm, 5 mm antero-medially of the umbo, and 

 bearing hollow embayments along the crests ; elongately oval ventral muscle scar 

 extending forward for 28% of the length of the pedicle valve ; cardinal process 

 ridge-like, simple. 



Description. Semi-elliptical, subequally biconvex Plectorthis, 3 pedicle valves 

 averaging 69% as long as wide (range 64% to 73%) and 22% as deep as long (range 

 21% to 24%) tending to become flatter away from the greatest depth of the valve 

 in the umbonal region, and an evenly convex brachial valve with a mean depth 

 relative to length of 19% (range 16% to 22%) for 3 valves ; anterior commissure 

 rectimarginate, cardinal angles slightly obtuse ; radial ornamentation evenly cos- 

 tellate with up to 27 flat-crested, parallel-sided costellae numbering 2 per mm, 5 mm 

 antero-medially of the umbo and arising by dichotomy of primary costae within 

 0-5 mm of the umbo, costellae becoming hollow at intervals of about 0-5 mm, 

 interspaces finely striate and imbricate ; ventral interarea apsacline, delthyrium 

 open, narrow, dorsal interarea shorter, anacline ; notothyrium open. 



Ventral interior with elongately oval muscle field averaging 74% (range 63% to 

 93%) as wide as long and 28% (no range) as long as the length of the valve for 3 

 pedicle valves, with well-developed submedial diductor lobes meeting antero- 

 medially to enclose a lanceolate undifferentiated adductor scar ; teeth small, 

 supported by short subparallel to narrowly divergent dental plates extending 

 anteriorly for an average of 9-5% the length of the valve for 2 valves. 



Dorsal interior with a simple ridge-like cardinal process, brachiophores short, 

 divergent, with bases slightly convergent to the floor of the valve and extending 

 anteriorly for an average of 10% of the length for 3 valves ; short fulcral plates 

 defining small suboval sockets, median ridge absent, adductor scars only rarely 

 impressed as a pair of widely spaced bilobed scars extending anteriorly for about 

 two-fifths the length of the brachial valve ; mantle canal systems unknown. 

 Type specimens. 



length width (mm) 

 Holotype External and internal moulds of pedicle valve 



(BB 35462a, b) 6-5 io-o 



Paratypes External and internal moulds of brachial valve 



(BB 35463a, b) 5-0 



External and internal moulds of brachial valve 



(BB 35464a, b) 5-0 8-5 



External and internal moulds of pedicle valve 



(BB 37158a, b) 6-o 9-2 



Horizon and localities. Whittery Shales exposed in the lower part of the 

 west bank of the River Camlad 60 yds north-east of Marrington Farm (Grid Ref. 

 SO 272970). 



Discussion. Many Ordovician Plectorthis have been described, especially from 

 North America (see Cooper 1956 : 447-456), but the species collected from the 

 Whittery Shales is unique in the combination of a number of distinctive features, 

 in particular the semi-elliptical outline and the peripheral flattening of the pedicle 



