52 SHELVE DISTRICT 



LENORTHIS Andreeva 1955 



Lenorthis cf. proava (Salter) 



(PI. 8, figs. 2-9) 



1866 Orthis calligramma var. proava Salter : 336. 



1868 Orthis Carausii [Salter ms.] Davidson : 315. 



1869 Orthis Carausii [Salter ms.] Davidson : 229. 



1869 Orthis calligramma var. proava Salter ; Davidson : 241. 

 1883 Orthis Carausii Salter ; Davidson : 182-184. 

 1968 Lenorthis proava (Salter) Bates : 146. 



Diagnosis. Semi-oval Lenorthis with a brachial valve 70% as long as wide and 

 17% as deep as long and a pedicle valve 31% as deep as long, ornamented by about 

 19 costae with a wavelength of 075 mm, 5 mm antero-medially of the dorsal umbo, 

 and fine capillae ; ventral muscle scar suboval extending anteriorly for 32% the 

 length of the pedicle valve. 



Description. Ventribiconvex, transversely semi-oval Lenorthis with an evenly 

 convex pedicle valve 31% as deep as long and a brachial valve 70% as long as wide 

 and 17% as deep as long with a shallow sulcus extending antero-medially of the 

 umbo for about 3 mm but becoming ill-defined thereafter ; ornamentation consisting 

 of 15 to 20 narrowly rounded costae on 1, 1, o, 1, 5 and 1 brachial valves more than 

 5 mm long, with a mean wavelength (and variance) of 0-75 mm (0-005), 5 mm anterior 

 of the umbones of 9 brachial valves, bearing delicate capillae with those occurring 

 medially in the interspaces tending to be stronger ; ventral interarea curved apsa- 

 cline, about one-seventh as long as the pedicle valve ; dorsal interarea shorter 

 anacline, cardinal extremities slightly acute or more rarely rectangular. 



Ventral interior with strong teeth supported by narrowly divergent dental plates 

 extending anteriorly for 17% of the length of the pedicle valve, pedicle callist small, 

 ventral muscle field subcordate, 96% as wide as long, with the submedial diductor 

 lobes slightly longer than the broad median adductor scar and extending anteriorly 

 for 32% of the length of the pedicle valve ; ventral mantle canal system rarely 

 impressed but with divergent vascula media. 



Dorsal interior with ridge-like cardinal process, oblique sockets and short, rod-like 

 brachiophores ; brachiophore bases extending anteriorly for about 14% of the length 

 of the brachial valve and 49% as long as the lateral extension of the simple sockets ; 

 dorsal adductor field quadripartite with the posterior pair of scars slightly larger, 

 extending forward of the umbo for 56% of the length of the valve. 



Figured material. 



Internal and external moulds of pedicle valve (BB 35501a, b) 

 Internal and external moulds of pedicle valve (BB 35502a, b) 

 Internal and external moulds of brachial valve (BB 35503a, b) 

 Internal and external moulds of brachial valve (BB 35504a, b) 



Horizon and localities. Mytton Flags : BB 35501, 35502 from exposures in 

 roots of felled tree 70 yds north of entrance to Yewtree Level, the Hollies, Snailbeach 



length 



width (mm) 



6-5 



77 



57 



8-o 



5-3 



8-o 



7'5 



- 



