Hinge teeth

Hinge teeth are part of the anatomical structure of the inner surface of a bivalve shell, i.e. the shell of a bivalve mollusk. Bivalves typically have two valves which are joined together by a strong and flexible ligament, and which in many cases articulate with one another using structures known as dentition or "teeth" which are situated along the hinge line. In most families of bivalves, the two valves of the shell are symmetrical with one another along the hinge line.

The hinge teeth are often very distinctive within a related group of bivalves, and thus examining the arrangement of the hinge teeth in a bivalve shell is often essential both for identification and for classification.

In taxonomy

The hinge teeth, or lack of them, is an important feature to use in identifying bivalves, because the teeth are generally similar within the major taxonomic groups. Historically the hinge teeth have provided a convenient means by which to construct classification schemes and attempt to indicate the phylogenetic relationships within the class Bivalvia. There are formal names for the various types of hinge tooth arrangements.

Kinds of hinge teeth

Taxodont

The taxodont hinge shows rows of similar interlocking teeth on either side of the umbones, as in the arc clams (Arcidae). Glycymerididae, and Nuculidae.

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  Arcidae hinge
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  Glycymerididae hinge
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  Nuculidae hinge

Dysodont

The dysodont hinge shows a strong ligament along the hinge line, with weak teeth near the umbones, as in the marine mussels (Mytilidae).

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  Mytilidae hinge

Isodont

The isodont hinge has lateral tubercles and sockets on either side of a thick ligament which is referred to as a resilifer. This arrangement is typical of the oysters (Ostreidae), scallops (Pectinidae), Spondylidae thorny oysters, and Plicatulidae kittens paws.

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  Ostreidae hinge
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  Pectinidae hinge
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  Spondylidae hinge
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  Plicatulidae hinge

Crurae

The crurae hinge has lamellar ridges on or near the hinge plate, and these function as hinge teeth. This arrangement is characteristic of the Pandoridae, Anomiidae, and Dimyidae.

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  Anomiidae hinge

Schizodont

The schizodont hinge has reverse "V" shaped scissurate teeth, and often an elongated lateral tooth. This arrangement is found in most Unionidae freshwater clams.

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  Unionidae hinge

Pachyodont

The pachyodont hinge has large obscure tubercules with corresponding pits on the opposite valve. This arrangement is characteristic of the Chamidae.

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  Chamidae hinge

Heterodont

The heterodont hinge has two to three wedge-shaped cardinal teeth set in the center near the umbones, and generally also has elongated lateral teeth on the anterior and posterior margins (on both sides, or sometimes only on one side). This arrangement is characteristic of the venus clams (Veneridae), cockles (Cardiidae) and several other important groups.

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  Veneridae: Pitar hinge
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  Veneridae: Tivela hinge
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  Veneridae: Dosina hinge
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  Cardiidae hinge

Desmodont

In the desmodont hinge, also known as an asthenodont hinge, the hinge consists of a large ligamentous resilifer (or chondrophore) which replaces the cardinal teeth, as in the soft-shell clams (Myidae).

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  Myidae hinge
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  Pholadidae hinge

Anodont

The anodont hinge is characterized by a strong ligament (or a series of transverse ligamental grooves as in the Isognomonidae), however true teeth are absent in adults, as is the case in the pen clams, tree oysters, and some freshwater mussels, e.g. Pinnidae, Pteriidae, Isognomonidae, and Anodonta.

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  Pinnidae hinge
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  Pteriidae hinge
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  Isognomonidae hinge