The nuculanid shell is small to medium-sized (to 70 mm), thin-walled, elongated oval, with the longer posterior end usually ROSTRATE. It is EQUIVALVE, compressed to gaping posteriorly at the siphonal opening, and INEQUILATERAL (umbones anterior), with ORTH-OGYRATE UMBONES. Shell microstructure is ARAGONITIC and two-layered, with an irregular PRISMATIC or HOMOGENOUS outer layer, and COMPLEX CROSSED LAMELLAR, CROSSED LAMELLAR, or homogenous inner layer. TUBULES are apparently absent. Exteriorly nuculanids are usually covered by yellow to dark brown, thin, varnishlike PERIOSTRACUM. Sculpture is smooth in some cases, but most often with predominantly commarginal (rarely scissuLATE) threads or ridges. LUNULE and ESCUTCHEON are present or absent. Interiorly the shell is non-NACREOus. The PALLIAL LINE is weakly impressed with a small to large PALLIAL SINUS. The inner shell margins are smooth. The HINGE PLATE is strong and TAXODONT, arched, with anterior and posterior series of chevron-shaped teeth separated by a subumbonal RESILIFER. The LIGAMENT is SIMPLE and submarginal, with an internal portion (RESILIUM).

Shell aragonitic, homogeneous. Inequilateral, elongate, posterior usually rostrate. Umbones anterior. Sculpture absent, or with weak commarginal lirae, rarely scissulate. Periostracum thin, varnished, usually yellow to dark brown. Hinge plate strong, arched, with two series of chevron-shaped teeth separated by resilifer or chondrophore. Ligament amphidetic, partially internal. Adductor muscle scars subequal, or with anterior larger. Pallial line weakly impressed; sinus small to large. Mantle lobes not fused ventrally, posteriorly forming siphonal openings. Gills protobranch, usually large. Hypobranchial glands absent. Labial palps long, narrow. Palp proboscides large. Foot with papillated planar sole; adult not byssiferous. Alimentary canal with type I stomach and long, convoluted intestine; style sac not joined to mid-gut; three ducts from stomach to digestive diverticula; rectum traversing through ventricle of heart.
A large and diverse family consisting of approximately 200 species assigned to three subfamilies found in all seas, but especially in deep water, usually in fine substrata of high organic content. The family has existed since at least the Devonian. All subfamilies are present in the northeastern Pacific.

Family Nuculanidae Shell round-ovate to elongate, equivalve, inequilateral, posterior often rostrated. Surface smooth or with concentric sculpture, covered with dark-olive to brownish, thin periostracum. Anterior adductor scar larger than the posterior one. Pallial sinus deep or shallow. Hinge arched, with anterior and posterior series of taxodont teeth. The members of the family are marine infauna animals and feed on detritus on the surface of substrates. 7 gen- era and 20 species are found in China seas.