148 SHELVE DISTRICT 



Type horizon and localities. Spy Wood Grit : BB 35363, 35365 from ex- 

 posures on top of ridge 1440 yds NNE of Rorrington bench mark 599 (Grid Ref. 

 SJ 303018) ; BB 35364, 35370 from outcrops 1100 yds NNE of Rorrington (Grid 

 Ref. SJ 303015) ; BB 35368, 35369 from the north bank of Spy Wood Brook, 170 yds 

 north-east of Spy Wood Cottage (Grid Ref. SO 282958) ; BB 35366, 35367 from 

 exposures 700 yds NNW of Middleton Church (Grid Ref. SO 296998). 



Discussion. As far as is known, no previously described species can be assigned 

 to Bystromena ; and even B. perplexa is represented by only a few moulds, mostly 

 imperfectly preserved, from the Spy Wood Grit so that much remains to be discovered 

 about the variability of the new species itself. 



Family LEPTAENIDAE Hall & Clarke 1894 



LEPTAENA Dalman 1828 



Leptaena cf. ventricosa Williams 



(PI. 27, figs. 5, 8, 9) 



Diagnosis. Leptaena with a semicircular disc about 10 mm long, ornamented 

 by relatively coarse costellae numbering 4 per mm, 5 mm antero-medially of the 

 umbo and about 9 low concentric rugae on the disc with a wavelength of 1 mm, sub- 

 circular ventral muscle scar less than two-thirds as long as the disc. 



Description. Concavo-convex, sharply geniculate and subquadrate Leptaena 

 with a semicircular disc about 10 mm long, almost twice as wide and about one- 

 eighth as deep in the vicinity of the ventral umbo, well defined by a confining fold 

 with a wavelength of 2 mm, trail sharply deflected at about 90 to give an overall 

 depth of about half the length of the pedicle valve ; ventral and dorsal interareas 

 apsacline and anacline respectively with a small pseudodeltidium truncated by a large 

 open supra-apical foramen and a well-developed chilidium ; radial ornamentation 

 finely but evenly costellate with 4 per mm, 5 mm antero-medially of the umbo ; 9 

 more or less continuous concentric rugae with a wavelength of about 1 mm also 

 ornament the disc. 



Teeth obliquely disposed and supported by short widely divergent dental plates ; 

 ventral muscle field not well preserved but apparently rhomboidal in outline and 

 about three-fifths as long as the disc. 



Cardinalia poorly preserved but a strong bilobed cardinal process, widely splayed 

 socket ridges and an anchor-shaped notothyrial platform were identifiable, as were 

 the transmuscle septa especially the longer, curved submedial pair which extend 

 anteriorly for about four-fifths of the length. 



Figured material. 



length width (mm) 

 External and internal moulds of brachial valve (BB 35453a, b) - - 



Internal mould of pedicle valve (BB 35454) - - 



Horizon and localities. Whittery Shales : BB 35454 from exposures in the 

 stream at the north end of Spring Coppice, 865 yds south-east of Hockleton Bridge 



