Fig. 6
a, b Oblongarcula cf. alemannica Owen, Altmann Member (Tierwis Formation). Silicified internal structures prepared by exposure to acid. a Dorsal valve interior from Tierwis near Grenzchopf, showing with compact hinge plate (NMSG P. 5193). b Juvenile specimen from Löchlibetter, showing a pair of dental lamellae with lateral umbonal cavities, in between initial stages of articulated hinge plates (horizontal arrow), crural bases with pointed ends of descending branches (vertical arrow), in the center a knob, probably the initial stage of median septum (oblique arrow) (NMSG P. 5194). Scale bars 5 mm