Shell small, approximately 10 mm in height, and 8 mm in length, somewhat convex and elongate of proportion. Anterior and posterior auricles subequal, umbonal angle about 85°. The valve has strongly developed irregular concentric lamellae on the entire external surface, curved on the auricles and near the anterior and posterior margin. Anterior auricle more curved than posterior, especially near the byssal sinus. Internal surface is opaque up to the pallial area, but near the ventral margin translucent, making the external concentric lamellae visible. Hinge line straight with broad triangled resilial pit. No cardinal crurae are exposed." (fide Dijkstra, 1990) Size: Normal adult size range is 8 to 10 mm.

Shell small, approximately 10 mm in height, and 8 mm in length, somewhat convex and elongate of proportion. Anterior and posterior auricles subequal, umbonal angle about 85°. The valve has strongly developed irregular concentric lamellae on the entire external surface, curved on the auricles and near the anterior and posterior margin. Anterior auricle more curved than posterior, especially near the byssal sinus. Internal surface is opaque up to the pallial area, but near the ventral margin translucent, making the external concentric lamellae visible. Hinge line straight with broad triangled resilial pit. No cardinal crurae are exposed. Holotype; one left valve-Measurements: height 9.1 mm, length 7.5 mm (ZMA Moll. no. 3.89. )9). Etymology. The species is named after Rear Admiral G. F. Tydeman (1858-1939), commander of H. M. SIBOGA.

Palliolum (Hyalopecten) graui (Knudsen, 1970) from the east Pacific is more circular. It has a finer sculpture of concentric lamellae, which are less pronounced and not strongly curved, and there is a delicate radiating striation on the external surface of the left valve.

Currently known only from Indonesia. Habitat: Deep water, muddy bottoms. Remarks: Only a few valves known.

Locus typicus. SIBOGA stat. 178. Indonesia, Ceram Sea, 2°40' S, 128°37’5 E. Deep sea trawl. 835 m. Blue muddy bottom. 2.IX.1899.