PLATE 5. 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. One articulated specimen preserved in ethanol. Fig. 1. Fresh articulated specimen preserved in ethanol and photographed with a Pentax camera K20D using a macroobjective (f. 50 mm). 1a: dorsal view. Shell is smooth, whitish-brown and trapezoidal foramen is much wider than strong pedicle. 1b: smooth ventral valve in ventral view. 1c: lateral view with suberect beak, distinct beak ridge, permesothyrid foramen and straight lateral commissure. Shell is dorsi-biconvex. 1d: anterior view; shell is clearly dorsi-biconvex. 1e: posterior view with permesothyrid foramen. Fig. 2. Articulated bleached specimen, SEM micrographs. 2a: dorsal view showing smooth dorsal valve, except for several distinct growth lines; large trapezoidal foramen bordered by two extremely narrow, disjunct, deltidial plates, flat palintropes and strong wrinkled pedicle collar. 2b: smooth external surface of ventral valve in ventral view. Strong irregular growth lines are visible. Fig. 3. Dorsal valve, critical point dried, exhibiting internal structures, especially the lophophore. 3a: ventral view showing outer epithelium (cut along commissure along valve), schizolophe lophophore constituting two rows of tentacles, and very large septum dividing the body chamber into two parts. Note tuberculation of ventral edge of septum. 3b: enlarged view of right branch of schizolophe lophophore. Note that outer epithelium covers lateral side of septum but not its ventral edge. 3c: detailed view of double rows of tentacles of lophophore. 3d: oblique anterior view. The relation between septum and epithelium is more clearly shown here. 3e: oblique lateral view showing near-complete partitioning of body chamber by septum. 3f: lateral view exhibiting clearly level of fixation of epithelium along septum and the irregular size of the tubercles ornamenting the ventral edge of the septum. Fig. 4. Ventral valve in dorsal view, critical point dried. 4a: complete valve in dorsal view. Outer epithelium cut along commissure and antero-medially a zone where it joins its dorsal part which is applied on lateral parts of septum. The four mantle canals are visible. In the posterior, pedicle and pedicle muscles are visible. 4b: detailed view of strong pedicle made of a thick sheath enclosing a bundle of anchorage papillae. 4c: detailed view of pedicle muscles.