70 SHELVE DISTRICT 



Astraborthis uniplicata gen. et sp. nov. 

 (PI. ii, figs. 5, 6, 9, ii, 14 ; Text-fig. 6) 



Diagnosis. Subcircular, dorsibiconvex Astraborthis with the pedicle valve over 

 four-fifths as long as wide and about one-third as deep as long ; ventral sulcus 

 rounded and about two-thirds as wide as the valve length, ornamented by angular 

 costellae with a wavelength of about 1 mm, 10 mm anterior of the ventral umbo with 

 4 on the fold and up to 16 on the shell ; pseudospondylium about one-third as long 

 as the pedicle valve. 



Description. Subcircular, strongly biconvex Astraborthis with the pedicle 

 valve over four-fifths as long as wide and about one-third as deep as long ; ventral 

 sulcus flanked by evenly convex lateral slopes, rounded in profile, about two-thirds 

 as wide as the length of the pedicle valve, projecting antero-dorsally to fit into 

 rounded dorsal fold ; brachial valve over one-third as deep as long, lateral slopes 

 strongly and evenly convex ; ventral interarea over one-quarter as long as the pedicle 

 valve, dorsal interarea about one-eighth as long as the brachial valve, cardinal 

 extremities obtusely rounded ; radial ornamentation coarsely costellate with delayed 

 costae originating at about 2 mm anterior of the umbo, with four occupying the crest 

 of the fold, ribs sharply angular with a wavelength of about i-o mm at 10 mm 

 anterior of ventral umbo. 



Ventral interior with pseudospondylium elevated on a solid platform of secondary 

 shell, about three-quarters as wide as long, lateral boundaries subparallel, anterior 

 boundary rounded extending forward from the ventral umbo for one-third the length 

 of the pedicle valve. 



Dorsal interior with plate-like cardinal process obtusely triangular in lateral view, 

 brachiophores short, curved, weakly divergent, adductor field strongly impressed 

 divided by moderately strong median ridge and bounded posteriorly by weakly 

 defined notothyrial platform. 



Type material. 



length width (mm) 

 Holotype Incomplete external and internal moulds of 



pedicle valve (BB 35324a, b) 10-5 12-5 



Paratype Incomplete external and internal moulds of 



brachial valve (BB 35325a, b) - 



Horizon and locality. Mytton Flags exposed in small pathside quarry in the 

 north-trending valley, 160 yds north-east of the north-east corner of Snailbeach 

 Reservoir (Grid Ref. SJ 378024). 



Discussion. Only the incomplete moulds of a brachial and a pedicle valve, 

 possibly the disarticulated remains of one shell, have so far been recovered from the 

 Mytton Flags but the generic and specific individuality of the specimens cannot be 

 doubted. Further collecting is desirable not only to assess the variation that 

 occurred within the species but also to determine whether dental plates were developed 

 in young specimens and to resolve the complete ribbing pattern, which, judged on 



