The yoldiid shell is small to medium-sized (to at least 60 mm), thin-walled, elongated oval, with the posterior end extended or slightly ROSTRATE. It is EQUIVALVE, usually compressed, usually gaping at both ends, and EQUILATERAL or INEQUILATERAL (umbones slightly anterior or posterior), with PROSOGYRATE UMBONES. Shell microstructure is ARAGONITIC and three-layered, with a PRISMATIC outer layer, a CROSSED LAMELLAR or HOMOGENOUS middle layer, and a COMPLEX CROSSED LAMELLAR or homogenous inner layer (in some species also with an innermost prismatic layer). TUBULES are apparently absent. Exteriorly yoldiids are covered by brownish to olive green, varnishlike PERIOSTRACUM. Sculpture is usually smooth and glossy, with fine commarginal growth lines. LUNULE and ESCUTCHEON are present or absent. Interiorly the shell is non-NACREOUS. The PALLIAL LINE has a deep SINUS. The inner shell margins are smooth. The HINGE PLATE is TAXODONT, arched, with anterior and posterior series of chevron-shaped teeth separated by a subumbonal RESILIFER. The LIGAMENT is SIMPLE, AMPHIDETIC, in some species submarginal set on FOSSETTES, and can be extended by periostracum; an internal portion (RESILIUM) is entirely mineralized, and splits in half dorsally with growth.

Shells inflated, thin, ovate to elongate. Ligament partly internal, in resilifer. Siphons present; pallial line with sinus. Adductor muscles subequal. Hypobranchial gland present. Labial palps large; palp proboscides narrow. Foot thick, with divided planar, papillated sole. Gills tranverse, with few gill plates. Alimentary canal consisting of large stomach and long, coiled intestine; style sac and mid-gut conjoined.
The family is known as early as the Cretaceous. Two subfamilies are present in the eastern Pacific.

YOLDIIDAE Allen and Hannah (1986) created the family Yoldiidae, which includes the genera Yoldia, Megayoldia, Cnesterium, Orthoyoldia, Yoldiella, Portlandia, Adrenella, Microyoldia, Ovaleda, and Sarepta. The yoldiids typically possess smooth, fragile valves that are compressed and elongate. The hinge teeth located on both sides of the umbo are semi-erect and separated by a chondrophore. Of all protobranchs Yoldia has been identified most often, mainly because of its relative importance in the food chain of commercial fish species; it causes bioturbation of sediments; and it is common, relatively large, and easy to handle (Reid 1998). The burrowing mechanism of Yoldia is most similar to that of Nuculana. This animal excavates a food chamber from which it feeds on bacteria. The excavation process creates sediments that disperse, causing bioturbation. The yoldiids possess inhalant and exhalant siphons, so they are able to use the ciliary action of their gills to suspension-feed (Reid 1998). In Texas Yoldiidae is represented by one species, Y. solenoides, whose size is approxi- mately 12 mm (½ in).