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The turtle shell is a shield for the ventral and dorsal parts of turtles (the order Testudines), completely enclosing all the vital organs of the turtle and in some cases even the head. [1] It is constructed of modified bony elements such as the ribs, parts of the pelvis and other bones found in most reptiles.
Crocodilian form of crurotarsal ankle. The astragalus (pink) is fixed to the tibia (green) by a suture. Adapted with permission from Palaeos. In the type of crurotarsal ankle, which is found in crocodilians and some other archosaurs, the astragalus is fixed to the tibia by a suture, and the joint bends around a peg on the astragalus, which fits into a socket in the calcaneum.
The ecdysial suture is longitudinally placed on the vertex, separating the epicranial halves of the head to the left and right sides. Depending on the insect, the suture may come in different shapes: like either a Y, U or V. Those diverging lines that make up the ecdysial suture are called the frontal or frontogenal sutures.
The following are the types of ventral sutures. [81] Connective sutures – are the sutures that continue from the facial sutures past the front margin of the cephalon. Rostral suture – is only present when the trilobite possesses a rostrum (or rostral plate). It connects the rostrum to the front part of the dorsal cranidium.
Sutures are closely spaced and slope forward from venter to dorsum with the obliquity increasing as growth progressed. The early part of the shell is essentially straight, but afterward expands and is notably curved. The early segments in the siphuncle [2] are bulb shaped and the septal necks are short. In the later part of the shell she ...
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The shell of the sea snail Epitonium scalare has a sculpture of sharp vertical ribs known as "costae" The shell of the freshwater snail Bulinus forskalii has strong transverse ribs which form an angle at the shoulder. Sculpture is a feature of many of the shells of mollusks.