BY WILLIAM A. HASWELL, M.A., B.SC. 73 



The following is a short general summary of the leading points 

 in the skeletal anatomy of the Plagiostomata. 



The vertebral column of the Plagiostomata varies considerably 

 in the degree to which the vertebra? become marked off from one 

 another, and the embryonic tissue becomes ossified. In some 

 ( Notidanus), no ossification takes place in the centra, and the 

 segments are not very clearly separated from one another. 

 In others ( Spinacidce, Lcemargidm and Echinorhinidce), each 

 centrum presents a double osseous cone with the apices meeting in 

 the middle and the cavities of the cones turned towards the 

 anterior and posterior faces of the vertebra.* In the greater 

 number of Sharks there is added to this double cone a series of 

 osseous rays traversing the cartilaginous zone which forms the 

 outer layer of the centrum. In one case only (Squatina) the 

 rays are replaced by a series of concentric lamella? surrounding 

 the double osseous cone. In some sharks (Notidanus for example), 

 each vertebra in the caudal region bears two neural arches. In 

 the Rays ossification of the vertebral column is more perfect than 

 in the Sharks, and the anterior portion of the spinal column 

 becomes fused into a continuous bony and cartilaginous mass. In 

 all the caudal vertebra? ai*e distinguishable by the presence of in- 

 ferior arches enclosing the caudal vessels. The first vertebra has its 

 anterior surface modified for articulation with the occipital region 

 of the cranium. In Hexanchus, in which the separation between 

 adjacent vertebra? is very imperfect, the cartilage of the first 

 vertebra is perfectly continuous with the cartilage of the cranium. 

 In most other Selachii, however, the first vertebra developes 

 lateral articular processes which articulate with the apposed 

 surfaces of the occipital regiou of the skull. These lateral articu- 

 lations are more markedly developed in the Rays, in which the 

 median prolongation of the centrum of the first vertebra? fits into a 

 deep excavation in the basis cranii, its apex being connected with 

 the latter by a mesial ligament containing the rudimentary 



* This structure seems first to have been noticed by Home : See " On the nature of the 

 intervertebral substances in Fish and Quadrupeds," Phil. Trans., 1809, pp. 177-184. 



