In Elasmobranchii the chordal sheath is weak and the skeletogenous layer strong. Biconcave cartilaginous vertebrae are developed, and as is the case in most fishes, constrict the notochord vertebrally.
Two distinct types of vertebral column can be distinguished in Elasmobranchs[43]:
1. In many extinct forms and in the living Notidanidae, Cestracion, and Squatina, the dorsal and ventral arches do not meet one another laterally round the centrum, and consequently readily come away from it.
2. In most living Elasmobranchs the arches meet laterally round the centrum.
The vertebrae are never ossified but endochondral calcification nearly always takes place, though it very rarely reaches the outer surface of the vertebrae. Elasmobranchs are sometimes subdivided into three groups according to the method in which this calcification takes place:
1. Cyclospondyli (Scymnus, Acanthias), in which the calcified matter is deposited as one ring in each vertebra.
2. Tectospondyli (Squatina, Raia, Trygon), in which there are several concentric rings of calcification.
3. Asterospondyli (Notidanidae, Scyllium, Cestracion), in which the calcified material instead of forming one simple ring, extends out in a more or less star-shaped manner.
In Heptanchus the length of the vertebral centra in the middle of the trunk is double that in the anterior and posterior portions, and as the length of the arches does not vary, the long centra carry more of them than do the short centra.
In many Rays the skull articulates with the vertebral column by distinct occipital condyles.