106 



PALAEONTOLOGY 



the dead fish, become scattered abroad by the movements of 

 the body through the action of the waters, and sink into the 

 sediment. 



Family I.— Cestraciontid^-:. 

 (Port-Jackson Shark.) 



The existing genus which has thrown most light upon the 

 fossil teeth which have thus become imbedded in the oceanic 

 deposits of the palaeozoic and mezozoic periods, is the Cestracion, 

 now restricted to the Australian and Chinese seas, where it is 

 represented by two or three species, and suggests the idea of a 

 form verging towards extinction. It formerly flourished under 

 a great number of varied generic or family modifications, re- 

 presented by species, some of which attained dimensions far 

 exceeding the largest known living Cestracions. The denti- 

 tion of these fishes is adapted to the prehension and mastica- 

 tion of crustaceous and testaceous animals ; they are of a 

 harmless, timid character ; and have the before-described den- 

 ticulate dorsal spines given to them as defensive weapons. 

 Fig. 27 gives a side view of the upper and lower jaws of 



the " Port Jackson shark," 

 showing the oblique disposi- 

 tion of the large crushing 

 teeth, which cover like a 

 pavement the working borders 

 of the mouth. The anterior 

 teeth were small and pointed. 

 Behind the cuspidate teeth 

 the five consecutive rows of 

 Fig. 27. teeth progressively increase 



Cestracion PhiUppi (recent). in all their dimensions, but 



principally in their antero-posterior extent The sharp 

 point is converted into a longitudinal ridge traversing a con- 

 vex crushing surface, and the ridge itself disappears in the 



