114 THE VERTEBRATE SKELETON. 



Cestracion, and Squatina, the dorsal and ventral arches do not 

 meet one another laterally round the centrum, and conse- 

 quently readily come away from it. 



2. In most living Elasmobranchs the arches meet laterally 

 round the centrum. 



The vertebrae are never ossified but endochondral calcifica- 

 tion nearly always takes place, though it very rarely reaches 

 the outer surface of the vertebrae. Elasmobranchs are some- 

 times subdivided into three groups according to the method 

 in which this calcification takes place : 



1. Cyclospondyli (Scymniis, 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. 



In BONY GANOIDS the skeletogenous layer becomes calcified 

 ectochondrally in such a way that the notochord is pinched in 

 at intervals, and distinct vertebrae are produced. Ossification 

 of the calcified cartilage rapidly follows. In Amia the verte- 

 brae are biconcave, in Lepidosteus they are opisthocoelous, cup 

 and ball joints being developed between the vertebrae in a 

 manner unique among fishes. The notochord entirely disap- 

 pears in the adult Lepidosteus, but at one stage in larval life 

 it is expanded vertebrally and constricted intervertebrally in 

 the manner usual in the higher vertebrata, but unknown 

 elsewhere among fishes. 



