LIVING MILLSTONES 163 



have gained a true idea of the dental armature of their 

 extinct relatives which abounded in the seas of the Jurassic 

 epoch. Visitors to Whitby must be familiar with certain 

 black oblong fossils of about an inch and a half in length 

 known to the quarrymen as " fossil leeches." These are 

 the hinder teeth of an extinct shark {Aster acanthus) nearly 

 allied to the Port Jackson species, but of much larger size ; 

 and although they are more rugose than pitted, they show 

 the same smooth line on the summit. A beautiful specimen 

 from Caen, in the British Museum, shows that the arrange- 

 ment of these hinder teeth was almost exactly the same 

 as in Cestracion, which may thus be regarded as a survivor 

 from a long-past epoch of the earth's history. 



But there were other " millstone-mouthed " sharks at a 

 still earlier period which appear to have been allied to 

 Cestracion, although the degree of relationship is uncertain. 

 In these Palaeozoic sharks, as exemplified by Cochtiodus, 

 the series of hinder teeth seems to have had an arrangement 

 very similar to that obtaining in Cestracion, but the indi- 

 vidual teeth of several series were more or less completely 

 fused into a single solid plate, the ridges on which mark 

 the original lines of division between the component series. 

 These sharks exhibit, therefore, one among many instances 

 where the earlier forms of a group are in some respects 

 more specialised than their descendants. 



So much space has been taken up by the rays and sharks 

 that only a few lines remain for the millstones of the enamel- 

 scaled fishes. In none of these do the teeth, which are 

 developed on most of the bones of both the upper and lower 

 jaws, ever form continuous plates ; and they are generally 

 either spherical or kidney-bean-shaped and arranged in more 

 or less longitudinal rows. Unlike those of the sharks and 

 rays, these teeth, as in the familiar Lepidotus of the Wealden, 



