146 SHELVE DISTRICT 



Ventral interior with obliquely placed teeth supported by short widely divergent 

 dental plates ; umbonal region of mature valves occupied by a short cylindroid 

 plug of secondary shell with a median groove on the dorsal surface ; ventral muscle 

 scar obscure, floors of mature valve with many radiating partitions suggesting a 

 lemniscate mantle canal system. 



Dorsal interior with delicate cardinalia consisting of a small bilobed cardinal 

 process with a vestigial median ridge present between the lobes in young specimens, 

 each lobe continuous with widely divergent socket ridges ; notothyrial platform 

 absent in young specimens so that the postero-medial part of the valve floor enclosed 

 by the cardinalia forms a hollow which is filled in mature valves by a low median 

 ridge bifurcating anteriorly ; muscle scars obscure with two pairs of transmuscle 

 septa identifiable in young specimens but masked in adult valves by strongly devel- 

 oped radiating partitions associated with a lemniscate mantle canal system. 



Type species. Bystromena perplexa sp. nov. from the Spy Wood Grit of the 

 Shelve area, Shropshire. 



Discussion. The shape of the shell, the disposition of the teeth and their supports 

 and the delicacy of the cardinalia indicate that the new genus is closely related to the 

 Glyptomeninae (Williams in Williams et al. 1965 : H388). Yet other features 

 immediately distinguish Bystromena from known members of that subfamily. They 

 include the tendency for the ribbing to be costellate rather than unequally parvi- 

 costellate, although the general pattern is like that of Platymena (Cooper 1956 : 879), 

 and the development of radiating partitions in the interior of adult shells. How- 

 ever, the most important distinction is undoubtedly the cylindroid mass of secondary 

 shell plugging the postero-medial part of the beak interior in adult shells. The 

 plug is in striking contrast to the well-developed foramen in young valves. There 

 are, moreover, indications that the anterior surface of the plug forms a hollow which 

 suggests that the structure was an internal pedicle tube, like those found in some 

 triplesiaceans (Wright 1963 : 744), but filled in with secondary shell following atrophy 

 of the pedicle. 



Bystromena perplexa sp. nov. 

 (PI. 25, fig. 14 ; PI. 26, figs. 3-14 ; PI. 27, figs. 1, 2 ; Text-fig. 11) 



Diagnosis. Transversely semi-elliptical Bystromena with a brachial valve about 

 three-quarters as long as wide and gently concave umbonally flattening peripherally 

 in maturity, and an evenly convex pedicle valve about one-tenth as deep as long ; 

 radial ornamentation finely costellate in adult shells with a modal count of 7 per mm, 

 5 mm antero-medially of the umbo. 



Description. Transversely semi-elliptical Bystromena with slightly acute to 

 obtuse cardinal angles, brachial valve about three-quarters as long as wide, slightly 

 concave in the umbonal region but flattening peripherally in adult growth stages, 

 pedicle valve evenly convex and about one-tenth as deep as long, protegulum large, 

 trilobed up to 1 mm long ; radial ornamentation commonly unequally parvicostellate 

 in early growth stages but becoming costellate in adult shells with counts of 5, 6, 7 



