ANATOMY OF VERTEBRATES. 



33 



Vertical transverse section of 

 centrum of Selache maxima 



fluent at their apices, which are perforated, and the notochord, 

 reduced to a beaded form, is continued through them : the 

 exterior of the bony cones is occupied by 

 a clear cartilage. In the Porbeagle Shark 

 (JLamna corniibica) further ossification of 

 the conical plate has reduced the central 

 communication to a minute foramen. Os- 

 seous plates have also been developed in 

 the exterior clear cartilage : these plates 

 are triangular, parallel with the axis of the 

 vertebra, their apices converging towards 

 the centre : the interspaces are filled by 

 cartilage. In the great Basking Shark 

 [Selache maxima) fig. 26, the longitudinal bony laminae are more 

 numerous and shorter than in Lamna, are peripheral in position, 

 and extend about one-third of the way towards the centre of the 

 interspace between the terminal cones, the rest being occupied by 

 a series of concentric cylinders of bone, interrupted by four 

 conical converging cavities, filled by cartilage ; of these, two, n, n, 

 are closed by the bases of the neurapophyses, and two, p, p, by 

 those of the parapophyses. There is a transition from the cylin- 

 drical to the longitudinally lamellar structure, the exterior and 

 largest of the cylinders sending out processes which join the in- 

 ternal margins of the converging lamellae In the Monk-fish 

 (Squatina) the osseous part of the centrum between the termi- 

 nal cones is entirely in the form of concentric layers, few in number, 

 and decreasing in breadth as they approach the centre. In the 

 Cestracion there are no concentric cylinders, but only longitudinal 

 Iamella3, radiating from the centre to the circumference, and giving 

 off short lateral plates as they diverge. 



In the Topes ( Galeus), the Blue Sharks ( Carcharias), and in 

 most sharks which possess the nictitating eyelid, may be seen the 

 most advanced stao-e of ossification in the cartilaginous fishes : 



o o 



the entire centrum, save at the four cavities closed by the neur- 

 and par-apophyses, is occupied by a coarse bone, more compact 

 where it forms the smooth exterior surface and that of the ter- 

 minal articular cavities. In osseous fishes (most Teleostomi) the 

 neur- and par-apophysial cavities are obliterated by bone, and 

 the neur- and par-apophyses are confluent, or suturally joined, 

 with the centrum ; but they retain a greater proportion, than in 

 higher classes, of the primitive gelatinous basis, which fills up the 

 deep cone or cup at each end of the centrum, fig. 27, c c. Only 

 in the ganoid Lepidosteus, among fishes, does ossification so extend 



VOL. i. 



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