The lyonsiid shell is small to medium-sized (rarely to 100 mm), very thin-walled, and irregularly oblong to orbicular, in some species posteriorly ROSTRATE, with the edges poorly calcified, and usually somewhat distorted due to the nestling habit. It is INEQUIVALVE (left valve larger and more convex, overlapping the right valve slightly), inflated anteriorly, compressed posteriorly, and can be gaping posteriorly for the siphons and in some cases ventrally for the byssus. The shell is INEQUILATERAL (umbones anterior), with PROSOGYRATE UMBONES. Shell microstructure is ARAGONITIC "PRISMATONACREOUS" and three-layered, with a simple PRISMATIC (or granular HOMOGENOUS) outer layer, a lenticular NACREOUS middle layer, and a sheet-nacreous inner layer. TUBULES are present, penetrating the inner and middle shell layers only. Exteriorly lyonsiids are white, often iridescent, covered by a thin to very thick, often radially arranged, dehiscent PERIOSTRACUM that often incorporates sand grains (variously interpreted as for increased camouflage, strength, or stability within soft sediment) and overlaps the free shell margins slightly. Sculpture is weak and variable, often with microscopic spines or nodules, in some cases with rugose commarginal wrinkles. LUNULE and ESCUTCHEON are absent. Interiorly the shell is thinly nacreous. The PALLIAL LINE is ENTIRE but is posteriorly weakly sinuate in some species. The inner shell margins are smooth. The HINGE PLATE is irregular and EDENTATE in adults. The LIGAMENT is internal (RESILIUM), OPISTHODETIC, set in a long, narrow RESILIFER, and reinforced by a short to elongated, more or less spatulate LITHODESMA. A secondary external ligament of fused periostracum unites the valves along the dorsal hinge margin.
The animal is ISOMYARIAN or HETEROMYARIAN (anterior ADDUCTOR MUSCLE smaller and more ventral); the posterior adductor muscle lies mid-dorsally, approximately halfway between the umbones and the posterior end of the shell. Anterior and posterior pedal retractor muscles are small. Pedal elevator and protractor muscles are absent. The MANTLE margins are extensively fused ventrally, usually with a small anteroventral pedal gape. Posterior EXCURRENT and INCURRENT SIPHONS are short and separate, but siphonal retractor muscles are poorly developed. Periostracum covers the base of the siphons and the ventral surface of the fused mantle edges. A FOURTH PALLIAL APERTURE is present posteroventrally. Arenophilic radial mantle glands (involved in adhering sand grains to the external shell surface) are present at the mantle edge (Lyonsia and juvenile Entodesma) or absent (Mytilimeria and adult Entodesma). Complex PALLIAL EYES are embedded in the excurrent siphon of some species of Lyonsia. HYPOBRANCHIAL GLANDS have not been reported. The FOOT is small (Entodesma) to large (Lyonsia), laterally compressed, unheeled, and has a BYSSAL GROOVE; the adult is byssate in some species.
The LABIAL PALPS are narrow and elongated. The CTENIDIA are EULAMELLIBRANCH (SYNAPTORHABDIC) and HETERORHABDIC; the outer demibranchs consist only of upturned descending lamellae. The gills are united posteriorly to the SIPHONAL SEPTUM, separating INFRA- and SUPRABRANCHIAL CHAMBERS. The gills are inserted into and fused with the distal oral groove of the palps (CATEGORY II association). Incurrent and excurrent water flows are posterior. The STOMACH is TYPE IV; the MIDGUT is coiled. The HINDGUT passes through the ventricle of the heart and leads to a freely hanging rectum. Lyonsiids are SIMULTANEOUS HERMAPHRODITES and produce large, yolky eggs that hatch LECITHOTROPHIC VELIGER larvae. The nervous system has not been well studied, but examination of available data suggests it is not concentrated. STATOCYSTS (TYPE Bl, with single large STATOLITHS) are known in adult Lyonsia. ABDOMINAL SENSE ORGANS have not been reported.
Lyonsiids are marine or estuarine SUSPENSION FEEDERS, in most cases shallowly and vertically INFAUNAL in fine sand (Lyonsia), or less often as nestlers (Entodesma) in rock crevices, algal holdfasts, or holes in sponges or tunicates. The Bladderclam (Mytilimeria nuttalli Conrad, 1837), from the northeastern Pacific, is totally sessile, living embedded in compound ascidian colonies.
The family Lyonsiidae is known since the Paleocene and is represented by ca. 9 living genera and ca. 45 species, distributed worldwide.