PLATE 6. Simplicithyris kurilensis Zezina, 1976, RBINS–INV. 146419 collected on August 13, 1972, off the eastern coast of Iturup Island (co-ordinates 44°37'N/146°57'E) in the Okhotsk Sea, Far East Russia, at a depth between 5 and 60 m. Bleached specimen with hypochlorite solution (5%). SEM micrographs. Fig. 1. Internal structures of ventral valve. 1a: dorsal view. Valve floor densely punctate and smooth, except for weak medial ridge forming triangular structure at bottom of valve. Teeth strong and bluntly pointed. Deltidial plates extremely reduced. Pedicle collar strongly developed. 1b: detailed view of posterior part of ventral valve. Detailed view of teeth, striated palintropes and of pedicle collar. This one has a wrinkled surface and is independent from ventral valve floor. Dental plates well developed, with a slightly concave aspect and a sharp, thin anterior edge. 1c: oblique anterior view of ventral valve showing pedicle collar and dental plates. 1d: posterior view showing eroded tip of beak, strong blunt teeth and weak medial ridge on valve floor. 1e: oblique lateral view of valve. 1f: detailed view of the punctuation. Fig. 2. Internal structures of dorsal valve. 2a: ventral view showing same dense punctuate shell and blade-like, septum with ventral protuberances. The very thick structure of posterior part of the shell is clearly visible as the very strong inner and outer socket ridges. 2b: detailed view of posterior part of valve showing thickened crural plates (arrowed "p") which form a wavy structure through growth. Small free spaces remain visible between crural plates and valve floor. 2c: oblique anterior view showing structure of septum and of crural plates. The wavy thick surface built by the crural plates through growth is well illustrated. 2d: oblique lateral view exhibiting blade-like septum with its cockscomb outline and showing 12 tubercles ornamenting its ventral edge. On the lateral side of the septum a wide convex ridge is developed. Two ridges join the posterior base of the septum and the dorsal bases of the crural plates which form a wavy thick surface through growth. Crura are absent. Sockets are very strong with thickened inner socket ridges. 2e: detailed oblique lateral view of the septum. Its cockscomb structure is well visible. The protuberances along its ventral edge are made of more or less regular tubercles except the anterior part where a larger tubercle is developed. The lateral sides of the septum are pinched with a wide ridge along their ventral part: these ridges are the zones where the ventral and dorsal epithelia are connected, forming two nearly isolated body chambers. 2f: oblique lateral view of the dorsal valve. 2g: Detailed oblique lateral view of the posterior part of the septum. 2h: detailed posterior view of septum showing that it is bifurcate and made of two lateral plates applied against each other. In this species the septum is hollow for a large part of its volume.