Yoldia is one of a group of "primitive" "Protobranch" bivalves which are different structurally from other bivalves. Most protobranchs live with the anterior end down in sediment. They have two small, bipectinate gills on the posterior end. Water is circulated from the anterior end toward the posterior end and across the gills, rather than in at the posterior ventral end, across the gills, and out at the posterior dorsal end as in most clams. The protobranchs usually have long extensions of the mouth called labial palps, which they extend into the sediment and pick up particles for feeding. Some Protobranchs, including Yoldia, also have a small flattened "sole" on their foot. The sole has left and right halves which can be folded together to collapse the foot into a narrow profile. The foot is then inserted into the sediment, the sole is unfolded to its wide configuration, and the foot is retracted to draw the clam down into the sediment.
Members of Subclass Palaeotaxodonta have equal valves with taxodont dentition and protobranch (bipectinate) gills.
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