18 



PISCES 



CLASS I 



purposes of classification. In the sharks and skates the pterygoquadrate 

 cartilage of the upper jaw is movably suspended from the cranium, and the 

 vertebral axis is often distinctly segmented ; in the Chimaeroids (Holocephali) 

 the upper jaw is fused with the cranium and the mandible articulates directly 

 with this, while the notochord remains unsegmented, so that the vertebrae 

 are always indicated only by the arches. 



Among the sharks (Plagiostomi) the Notidanidae exhibit the most 

 primitive vertebral axis (Fig. 31). Here regularly spaced thickenings of the 

 notochordal sheath constrict the chord itself and form divisions pierced 

 through the middle (Diplospondyli). The term Cyclospondyli is applied by 

 Hasse to those vertebrae in which a calcified ring appears uniting the 



d 



cJu-S 



Fig. 31. 



~ Longitudinal section of the anterior part of the 

 vertebral column of Heptanchus. eh, Notochord ; 

 d, Incipient calcified double -cone (basis of 

 centrum) ; iv, Intervertebral space filled with 

 notochord ; w, Constriction of notochordal sheath 

 (after Kolliker). 



,.eh 



Longitudinal section of the tectospon- 

 dylic vertebral column of Squatina 

 angelus, Linn, eh, Notochord : </, Cal- 

 cified double -cone (basis of centrum); 

 iv, Intervertebral space ; v, Vertebral 

 centra with concentric calcified rings 

 (after Hasse). 



E~ 



E- 



ordinarily calcified anterior and posterior borders of the primitive constricted 

 centrum (Fig. 33, A). "When additional calcified lamellae concentric with the 

 first are developed in the intravertebral space, the Tectosjjondt/li (Figs. 32, 33, B) 

 are recognised ; when, on the other hand, calcified plates or ridges radiate 



from the centre to 

 the periphery, the 

 vertebrae are 

 termed asterospon- 

 dy lie, and the group 

 Asterospondyli (Fig. 

 33, C). 



With the verte- 

 brae there occur 

 most abundantly 

 in the fossil state 

 teeth, fin spines, 

 and occasionallv 



*J 



dermal plates. 

 Sharks' teeth are 

 among the fossils 



which have been longest known, and are frequently mentioned in the older 

 writings under the names of glossopetrae, birds' tongues, and snakes' tongues. 

 The form of the teeth is very variable, often sharply pointed with cutting edges, 

 single- or many-cusped, but frequently also pavement-like, with a flattened, 

 bluntly conical crown. They consist essentially of vasodentine, dentine, and 

 structureless enamel (ganodentine). Except perhaps in some extinct Acan- 

 thodii, they are always fixed by ligament, never fused with the supporting jaw. 



-B 



-N 



D 



H 



Fig. 33. 



Diagrammatic vertical section through the middle of Selachian vertebrae of the 

 mdylvi(A), tectosporulylic(B), and asterospondylic (C) Jjypes. C, Space for 



notochord; D, Central "calcified "ring or double cone; E,' Elastica externa 

 Haemal arch ; N, Neural arch (after Hasse). 



H. 



