Shells almost always equivalve, inequilateral, elongate, subtrigone to subcylindrical, with acute summits and often a ventral byssal slit. Hooks close to the anterior extremity, prosogyrate; hinge often edentulous or bearing some weak serrations; ligament almost internal, posterodorsal. Posterior adductor muscle scar well developed. Inside of the vavles nacreous. Sculpture variable.
 

Shell medium-sized, « equivalve, almond-shaped, usually compressed a little at the front and therefore wider, angular at the back and usually gaping at the edge. Hinge terminal… » – J. C. Megerle von Mühlfeld: “Entwurf eines neuen Systems der Schaltiergehäuse”, Magazin für die neuesten Entdecklungen in der gesammten Naturkunde von der Gesellschaft Naturforschaft Freunde zu Berlin vol.V(1), Berlin 1811, p.69.
 

Shell small, equivalve, inequilateral. « Dr. Jousseaume places the Mod. arcuatula in a new genus Arcuatula, created for the Modiola group, in which almost all the species are arched and whose typical form is that of M. plicatula Lamarck [= demissa Dillwyn]; the shells of this genus, some of which are almost smooth and others of which are strongly striated, have the edge of the ligament very long, and the angle formed by this edge and the posterior one is blunt and often rounded; in the genus Brachydontes, on the contrary, the edge of the ligament is short, the angle more salient and the posterior edge often very long and arched inside. » – E. Lamy: “Les Moules et les Modioles de la Mer Rouge (fin)”, Bulletin du muséum national d’histoire naturelle vol.XXV, Paris 1919, p.174.
 

Shell small, equivalve, inequilateral. « Umbones prominent, not terminal; valves corrugated; hinge margin considerably angulated; teeth many, small, and crenate. » – W. Swaison: A treatise on malacology or shells and shell-fish, London 1840, p.384.
 

Shell minute, ovate, equivalve, equilateral, thin, pellucid. Valves convex, with umbones prominent. Hinge short and crenulate. Adductor muscle scars of almost equal size (the genus is isomyarian).
 

«  The Dacrydiinae are neotenous mytilids, many being associated with sponges to which they are byssally attached. Multiple fine byssal threads are produced, which may form a nest. The shells are small, rarely more than 4mm total length, fragile, and transluscent, white or pale cream in colour. Ornamentation, when present, consists of fine concentric striae, sometimes with very fine radial lines and occasionally a scattering of tiny shell granules. The hinge is narrow, with fine, multiple, nepioconch teeth retained throughout life. The ligament is small, internal, and amphidetic. Ventral to the posterior hinge plate is a shell buttress parallel to the dorsal shell margin, which probably provides necessary strength to an extremely fragile shell at times subject to adduction. » – J. A. Allen: “The deep-water species of Dacrydium Torell, 1859 (Dacrydiinae: Mytilidae: Bivalvia), of the Atlantic”, Malacologia vol.40, Ann Arbor 1998, p.1.
 

Shell in which « one third is smooth and two thirds decussate and decorated with bearded epidermis; anterior part of the hinge with rudimentary oblique folds; ligament crenellated. Gregarious. » – T. A. di Monterosato: “Conchiglie littorali mediterranee”, Naturalista Siciliano anno 3, Palermo 1883-1884, p.90.
 

Shell often large, slender, thin, fragile, with curved summits very anteriorly placed; ligament internal; hinge reduced to a simple line bearing posteriorly a long whitish filament, and anteriorly a thin lamella; muscular prints extremely weak. The genus is named after Mr. Joly, « who the first discovered it… » – J. R. Bourguignat: “Descriptions de deux nouveaux genres algériens”, Bulletin de la Société des Sciences Physiques et Naturelles de Toulouse vol.III, Toulouse 1877, p.53.
 

Shell medium-sized, inequivalve, inequilateral. « Animal penetrating deeply inside its matrix, in which it excavates a polished chamber where it resides… » – P. P. Carpenter: Catalogue of the collection of Mazatlán shells in the British Museum, London 1857, p.130. « Smooth shells with smooth chalky covering extending beyond the tips of the valves. » – T. Iredale: “Mollusca” part I, Scientific Reports / Great Barrier Reef Expedition 1928-29 vol.V, London 1939, p.40.
 

Shell small, with a glossy periostracum; dorsal and ventral margins almost parallel; Like in Modiolus, there is a kind of transverse hump than runs from the summits to the posteroventral area; « radial sculpture obsolete or absent; branchial siphon equal or nearly equal to the anal, both much elongated. » – W. H. Dall: Le règne animal vol.II, Paris 1817, p.471.
 

Shell medium-sized to large, equivalve, inequilateral. Synonym Lithodomus Cuvier, who writes: « …oblong shell, almost equally rounded at both ends, and the summits very close to the anterior end. They first hang on stones, like common mussels, but then they pierce them to introduce themselves, and dig cavities, from which they no longer leave. Once they have entered, their byssus no longer increases. » – G. Cuvier: Le règne animal vol.II, Paris 1817, p.471.
 

Shell small, oblong, equivalve, inequilateral, tumid; strong periostracum made of scattered scales and hair; the beaks, small and blunt, are very close to the anterior; hinge thick and finely crenulated; posterior and anterior margin almost parallel, pallial margin rounded; byssal sinus visible; muscular impressions not discernable.
 

Shell medium-sized to large, equivalve, inequilateral, « subtransverse, with extremely short posterior side, and hooks tilted towards the short side of the shell; a single muscle impression; hinge simple, toothless. » – J. B. M. de Lamarck: “Prodrome d’une nouvelle classification des coquilles”, Mémoires de la Société d’Histoire Naturelle de Paris vol.I, Paris 1799, p.87-88. The dorsal and ventral regions are separated by a kind of bevelled crest that becomes like a « step (rounded) in the anterior part of the shell » (Brusina).
 

Shell small, thin, with very convex valves, « rhomboidal, sculptured by two rows (one on each side) of striae which radiate from the beaks, leaving the middle portion smooth; beaks incurved; hinge mostly toothless, but sometimes crenulated; hinge-plate finely notched. This genus differs from Mytilus in the mantle being folded into a distinct tube for the supply of food and aerated water, as well as in the remarkable sculpture of the shell. » – J. G. Jeffreys: British conchology vol.II, London 1863, p.120-121, about the genus Modiolaria Beck, synonym.
 

« Ordinarily small species with rippled sculpture and strong growth marks; ventral margin sinuous; hinge with corresponding teeth and cavities; the whole ligamentary side distinctly denticulated. » – T. A. di Monterosato: Nomenclatura generica e specifica di alcune conchiglie mediterranee, Palermo 1884, p.9-10.
 

Shell medium-sized to large, dark, equivalve, inequilateral; « umbones terminal or subterminal. Cardinal teeth few, small, sometimes obsolete. Anterior adductor muscle sometimes wanting. […] Surface smooth, or concentrically sculptured… » – H. V. Ihering: “On the South American species of Mytilidae”, Proceedings of the Malacological Society of London vol.IV, London 1900, p.86.
 

« Nypamodiolus n. gen. is characterized by small-sized, modioliform to fan-shaped shells with the umbones in a non-terminal position, and by robust anterior and posterior byssal retractor muscles arranged in roughly one plane. » – Lin, Kiel, Xu & Qiu: “Phylogenetic placement, morphology and gill-associated bacteria of a new genus and species of deep-sea mussel (Mytilidae: Bathymodiolinae) from the South China Sea”, Deep Sea Research Part I vol. 190, December 2022, abstract.
 

Shell medium-sized to large, equivalve, inequilateral; periostracum brown on a whitish surface. « The anterior adductor is always present in Mytilus, absent only in older Aulacomya specimens, and completely absent in Choromytilus and Perna. There is a single retractor muscle scar in Mytilus and Aulacomya, one and occasionally two in Choromytilus, and two in Perna. […] Perna, having lost the anterior adductor muscle, divided the retractor muscle complex, and developed primary lateral hinge teeth and branching papillae on the mantle margin […] The presence of primary lateral hinge teeth allows us to differentiate larvae of Perna from those of Mytilus. Adults of the genera are most reliably distinguished by patterns of muscle scars. » – S. E. Siddall: “A clarification of the genus Perna”, Bulletin of Marine Science vol.XXX, Miami 1980, p.867-868.
 

Shell small, thin, equivalve, very inequilateral, with convex valves. « Sculpture divaricate covering the entire surface. Hinge with two conspicuous teeth; denticulate ligament line; margin crenulate. » – T. A. di Monterosato: Nomenclatura generica e specifica di alcune conchiglie mediterranee, Palermo 1884, p.13. Prodissoconch very large, adorned with a different sculpture than on the adult parts.
 

Shell small, equivalve, very inequilateral, « with a subconcave ventral edge indented for the passage of the byssus. […] Hinge toothless, provided with a lamellar and vertical partition inside the hooks. Ligamentary spatulae linear, marginal, dorsal. […] The shells of the genus Septifer are very compressed at the ventral edge in the young age, gradually become concave while advancing towards adulthood, and the edges of this side are notched so as to leave a space large enough for the byssus to act freely. […] finally the surface of the valves is engraved with small grooves which split towards the center. » – C. A. Récluz: “Description d’un nouveau genre de coquille bivalve nommé Septifère (Septifer)”, Revue Zoologique vol.XI, Paris 1848, p.278.
 

Shell small to medium-sized, equivalve, inequilateral, elongate, sub-cylindrical or arcuate, brownish. « Umbonal keel prominent; […] exterior, including the lunule, smooth with radial striae (but sometimes with weak concentric growth striae). Umbones terminal or almost terminal. Ligament moderately strong or weak, resilial ridge compact. Hinge line entirely edentulous except for a peg-like, posteriorly pointing projection below the umbo. Periostracum thin, smooth and shiny&hellip: »– B. R. Wilson: “A new generic name for three recent and one fossil species of Mytilidae (Mollusca: Bivalvia) in southern Australasia with re-descriptions of the species”, Proceedings of the Malacological Society of London vol.XXXVII, London 1967, p.281.