Bivalve hinges may have structures called teeth and sockets (or dentition) that help to align the valves as they close and make it harder for a predator to open the shell by twisting the valves. Various types exist and are useful in classifying and identifying taxa.
The most primitive dentition is called taxodont from two Greek roots meaning "row" (taxos) and "tooth" (dontos). The name refers to the roughly straight rows of roughly equal sized teeth, as shown in the three species below (from left to right: Nuculana aegeensis, Carter Coll. RGa 907 -- Jacksonville, FL; Anadara ovalis, Carter Coll. RGa 59, also from Jacksonville, and Glycymeris glycymeris Carter Coll., Biarritz, France). This state is thought to have evolved twice: once early in bivalve evolution in a taxon called the Paleotaxodonta (Nuculana, for example) and then again in a separate taxon called the Arcoida (the other two genera below).
In some cases there is no dentition, a condition called edentate or edentulous. Oysters and scallops are edentulous with a resilifer that contains the internal ligament. The two photos below show this clearly. The oyster looks like it has one big, wide socket for a tooth, but the other valve usually also has a "socket". The resilium fits into these depressions on both valves. (In some oysters there may be a slight projection on one valve, but this is not a tooth because the ligament attaches to its surface as well and keeps it from articulating with the depression on the other valve.) In the scallops the resilifer is smaller and occupies a distinct pit on each valve, near the middle of the hinge. Note that the asymmetry of the scallop shell is even more evident on the inside: the single (posterior) adductor and the shape of the junction between the anterior ala and the body of the valve are where it is most apparent.
Certain close relatives of scallops of the family Plicatulidae, like Plicatula sponduloides as shown below at left (Carter Coll., RGa856, Cape San Blas, FL), also have a resilifer, but this is flanked by a pair of identical teeth. This dentition is called isodont. (In Greek, isos means "same").
Other edentuolous hinges, as in many mytiloids, lack a resilifer. The picture above at right shows Modiolus auriculatus (Carter Coll., RGa 163 -- Nuweiba, Egypt) with no teeth and with a long ligament rather than an equidimensional resilium. Other mytiloids have one or several very weak, nearly useless teeth and an elongate ligament, as in Mytilus edulis shown below at left (Carter Coll., RGa458 -- Cape Henlopen, DE). This condition is called dysodont dentition, from the Greek dysos, meaning "poor" in the sense of "ill-formed" or "not right").
The most advanced types of bivalves have teeth of two different types, as shown in Mercenaria above at right. The name of this dentition is heterodont from the Greek root heteros, meaning "different". Near the center of the hinge a pair of relatively short teeth (viewed straight on) cross the hinge at roughly a right angle and project quite far outward. These are the cardinal teeth. Anterior and/or posterior of these are longer, lower teeth that run more nearly parallel to the hinge. These are the lateral teeth. In the simpler form there is an elongate ligament above these; in the more complex form the ligament is a small, roundish resilium that is held in a structure that projects from the hinge some distance, looking like a strangely shaped tooth. This is called the chondrophore, and is shown in a myoid clam below left (Mya arenaria Carter Coll., RGa 472 -- Popham Beach, ME) and a mactroid clam below right (Spisula solidissimus, Carter Coll., RGa 373 -- Assateague Island, MD). Dentition of the more complex type, with the chondrophore, is called desmodont, though not everyone bothers to distinguish it from ordinary heterodont dentition.
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