Shell suborbicular, equivalve, nearly equilateral, convex, with a velvety periostraeum; umbos central, nearly straight; ligament external; cardinal border regularly arched or semi-circular; hinge with a semi-circular row of numerous, prominent, strong, oblique teeth, which become interrupted during growth by the subsidence of the area margin; margins crenulated within; adductors subequal; pallial line simple. Animal with the mantle open throughout; foot crescent-shaped, without byssus, but with a plantar surface; the heart traversed by the rectum.
Loc.: Tropical and subtropical seas.
Remarks: Fossil, Cretaceous; maximum species in the Miocene.

The glycymeridid shell is small to medium-sized (to 100 mm), circular to oval to rounded trigonal, in some cases slightly oblique, and solid. It is EQUIVALVE, moderately inflated, and not gaping. The shell is EQUILATERAL, with the UMBONES central, ORTHO-, occasionally OPISTHO-, or rarely PROSOGYRATE, and separated by a narrow to wide CARDINAL AREA. Shell microstructure is ARAGONITIC and two- or three-layered, with a PRISMATIC outer layer (absent in some species), CROSSED LAMELLAR middle layer, and COMPLEX CROSSED LAMELLAR inner layer. TUBULES have been reported for some species through all shell layers except the outermost (PERIOSTRACUM). Exteriorly glycymeridids are creamy white or brown covered by a usually thick, velvety, dehiscent periostracum that is frequently hirsute in smoother-shelled species. Sculpture is smooth to strongly radial; Glycymeris and Tucetona genus groups
have been recognized based on fine or strong ribs and corresponding hirsute or smooth periostracum, respectively. LUNULE and ESCUTCHEON are absent. Interiorly the shell is non-NACREOUs; the posterior (in some cases also the anterior) adductor muscle scar has a raised ridge (MYOPHORIC RIDGE) on the inner margin. The PALLIAL LINE is ENTIRE. The inner shell margins are denticulate, with denticles corresponding to the external spaces between ribs. The HINGE PLATE is robust and strongly arched, TAXODONT, with numerous radially arranged, in some species chevron-shaped teeth (the larger of which are commonly crenulate), diminished in size below the umbones and laterally. The LIGAMENT occupies
The LABIAL PALPS are relatively small, and are extended by a long, unridged oral groove to the mouth. The subumbonal CTENIDIA are FILIBRANCH (ELEUTHERORHABDIC), HOMORHABDIC, of about equal size, and not inserted into (or fused with) the distal oral groove of the palps (CATEGORY III association). CEPHALIC EYES are present. Incurrent and excurrent water flows are posterior, with a secondary anterior incurrent. The STOMACH is TYPE III. The MIDGUT is not coiled and is somewhat enlarged in diameter. The HINDGUT passes through the ventricle of the heart, and leads to a freely hanging or sessile rectum. Hemoglobin has been found in the blood of some species. Glycymeridids are generally GONOCHORISTIC, although evidence suggests that at least one species is a PROTANDRIC HERMAPHRODITE, and produce planktonic VELIGER larvae. The nervous system is apparently not concentrated. STATOCYSTS have not been reported in adults. ABDOMINAL SENSE ORGANS are present and asymmetrical.
Glycymeridids are SUSPENSION-FEEDING, shallow INFAUNAL burrowers just below the surface of coarse sand. Some species show intolerance of silt and turbid water. Atlantic species are slow, inefficient burrowers that are believed to be active exclusively at night.
The family Glycymerididae is known since the Cretaceous, is represented by 4 living genera and at least 50 species, and is distributed worldwide except in polar and deep seas. Glycymeris is commercially fished in parts of Europe and the Mediterranean; the common name "bittersweet clam" undoubtedly refers to their taste.

Shell aragonitic, with crossed-lamellar outer and complex crossed-lamellar inner layers; interior porcelaneous. Outline subequilateral, circular to subtriangular; equivalve, thick and heavy in many. Umbones prominent, orthogyrate or prosogyrate, rarely opisthogyrate. Cardinal area narrow to wide. Periostracum frequently hirsute. Hinge plate broad, arched; taxodont teeth in two series. Ligament external, amphidetic or prosodetic, duplivincular. Inner ventral shell margin crenulate. Dimyarian, with subequal adductor muscles. Mantle lobes not fused; siphons absent. Mantle border with ocelli. Ctenidia filibranch, eleutherorhabdic. Lips simple. Labial palps small. Foot large, wedge-shaped. Alimentary canal with type 3 stomach, with multiple ducts leading to digestive diverticula; style sac conjoined to mid-gut; intestine passing through ventricle of heart. The family, known from the Cretaceous, contains one living subfamily of robust, rounded shells, shallow burrowers in sand and other sediments with large particles, mostly in warm, shallow seas. We appreciate the advice of Charles L. Powell, II, and Akihiko Matsukuma on this family.