HETERODONTI 143 





FAFI^Y NOTIDANIPVE. These typically amphistylic sharks have a 

 more normal Selachian structure, and have diverged chiefly in their 

 dentition. In modern genera the teeth are very unlike in the two jaws. 

 In the lower jaw they are long, very compressed, and with a saw-like 

 edge (Fig. 102). The gradual differentiation of this type of tooth, from 

 one with a broad base supporting a large cusp and a few small ones, and 

 closely resembling that of Hybodus (p. 144), can be traced in the fossil 

 genus Notidanus (A. Smith Woodward [505]). The upper teeth have 

 departed much less from the primitive type. 



The fact that in the Jurassic N. eximius, Wagn., the centra are more 

 calcined even than in Heptanchus points to the modern forms bein^ 

 somewhat degenerate. 



Notidanus, Cuvier ; Jurassic and upwards, Europe and New Zealand. 

 Heptanchus, Raf.; Atlantic and Mediterranean Seas. Hexandiu*, Raf. ; 

 warm and tropical seas. 



GROUP II. 



In these Selachians the number of branchial slits does not 

 exceed five (except in Pliotrema, p. 152); the notochord becomes 

 much constricted by usually well -developed centra; the occipital 

 region of the skull is clearly marked off from the column, and 

 the occipito-spinal region very much shortened ; a cartilaginous 

 floor is formed below the orbit, except in the Rajiformes. The 

 external branchial openings are, as a rule, considerably diminished 

 in length by the overgrowth of the skin above and below. 



An anal fin is present, except in the Subdivision 2 (p. 151, 

 Squaliformes and Rajiformes), where it appears to have been lost. 

 There are two dorsal fins. 



DIVISION A. 



With little or no rostrum and no triradiate supporting cartilage, 

 the mouth being almost terminal. The attachment of tne jaws of 

 the early forms is amphistylic; that of the later forms hyostylic. 



Sub-Order 1. HETERODONTI. 



An ancient sub -order dating back beyond the Carboniferous 

 epoch* probably to the Devonian, but surviving at the present day 

 in one genus only, Heterod&ntus (Cestracion). In this modern 

 Port Jackson shark the notochord is much constricted by well- 

 developed asterospondylous centra; the jaws are hyostylic, but 

 with a very extensive palato-basal articulation, so that the 

 hyomandibular scarcely acts as a real support (Fig. 58). The 

 teeth of Heterodontus are highly modified in both jaws, those 

 towards the centre being small and pointed, those towards the 

 sides being blunt and flattened into crushing plates. Several 

 rows of teeth are functional at the same time (Fig. 107). 



