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 by definition have two valves, which are joined together by a strong and flexible ligament situated on the hinge line at the dorsal edge of the shell. In life, the shell needs to be able to open slightly to allow the foot and siphons to protrude, and then close again, without the valves moving out of alignment with one another. To make this possible, in most cases the two valves are articulated using an arrangement of structures known as hinge teeth (often referred to collectively as the "dentition"). Like the ligament, the hinge teeth are also situated along the hinge line of the shell, in most cases.
In most families of bivalves, the two valves of the shell are almost perfectly symmetrical with one another along the hinge line, although the placement and shape of the teeth may differ slightly in the left valve and right valve in order for the two valves to articulate properly.
Each related group of bivalves tends to have distinctive hinge teeth, and because of this, examining the arrangement of the hinge teeth in a bivalve shell is often essential for identification and classification.
A formal terminology is used to describe the different types of dentition.
In taxonomy
The hinge teeth, or the lack thereof, is an important feature 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.Sturm, C. F., T. A. Pearce, and A. Valdes. 2006. The Mollusks: A guide to their Study, Collection, and Preservation. American Malacological Society, Pittsburgh, PA, U.S.A. xii+455 Pp.Huber, Markus (2010). Compendium of Bivalves. A Full-color Guide to 3'300 of the World's Marine Bivalves. A Status on Bivalvia after 250 Years of Research. Hackenheim: ConchBooks. pp. 901 pp. + CD. {{ISBN|978-3-939767-28-2}}, at p. 59Bivalves by J.H. Leal, Bailey-Matthews Shell Museum, Florida, USA {{cite web |url=http://www.shellmuseum.org/BivalvesLeal.pdf |title=Archived copy |access-date=2012-11-13 |url-status=dead |archive-url=https://web.archive.org/web/20130511085317/http://www.shellmuseum.org/BivalvesLeal.pdf |archive-date=2013-05-11 }}Invertebrate Paleobiology on-line syllabus on Bivalves, by Dr. Burt Carter, Georgia Southwestern State University, at: http://itc.gsw.edu/faculty/bcarter/paleo/labs/moll/biv2.htm {{Webarchive|url=https://web.archive.org/web/20200225231137/http://itc.gsw.edu/faculty/bcarter/paleo/labs/moll/biv2.htm |date=2020-02-25 }} There are formal names for the various types of hinge tooth arrangements or dentition.
Kinds of hinge teeth
=Taxodont=
The taxodont hinge shows either one or two rows of similar interlocking teeth on either side of the umbones, as is the case in the ark clams (Arcidae), the bittersweets (Glycymerididae), and the nut clams (Nuculidae).
File:Arcidae hinge.jpg|Arcidae hinge
File:Glycymerididae hinge.jpg|Glycymerididae 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).
File:Mytilidae hinge.jpg|Mytilidae hinge
=Isodont=
The isodont hinge has lateral tubercles and sockets on either side of a thick ligament which is referred to as a resilium. This arrangement is typical of the oysters (Ostreidae), scallops (Pectinidae), thorny oysters (Spondylidae), and kittens paws (Plicatulidae).
File:Ostreidae hinge.jpg|Ostreidae hinge
File:Pectinidae hinge.jpg|Pectinidae hinge
File:Spondylidae hinge.jpg|Spondylidae hinge
File:Plicatulidae hinge.jpg|Plicatulidae hinge
=Crurae=
The {{lang|la|crurae}} hinge has lamellar ridges on or near the hinge plate, and these function as hinge teeth. This arrangement is characteristic of the Pandoridae, the jingle shells Anomiidae, and Dimyidae.
File:Anomiidae hinge.jpg|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 mussels.
File:Unionidae hinge.jpg|Unionidae hinge
=Pachyodont=
=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 one or both sides). This arrangement is characteristic of the venus clams (Veneridae), the cockles (Cardiidae) and several other important groups.
File:Veneridae close up heterodont hinge.jpg|Veneridae: Pitar hinge
File:Veneridae hinge.jpg|Veneridae: Tivela hinge
File:Veneridae hinge 3.jpg|Veneridae: Dosina hinge
File:Cardiidae hinge.jpg|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).
File:Myidae hinge.jpg|Myidae hinge
File:Pholadidae hinge.jpg|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 shells (Pinnidae), tree oysters (Isognomonidae), the pearl oysters (Pteriidae), and some freshwater mussels such as the genus Anodonta.
File:Pinnidae hinge.jpg|Pinnidae hinge
File:Pteriidae hinge.jpg|Pteriidae hinge
File:Isognomonidae hinge.jpg|Isognomonidae hinge