CESTRACIONTS, 63 



straight, sub-parallel course, and sparing dichotomous sub-divisions, 

 but they are relatively wider, and are separated by interspaces of 

 less breadth. 



The calcigerous tubes are also wider at their origin ; they come 

 off not quite at right angles to the medullary canal, but are slightly 

 inclined to the grinding surface of the tooth ; they quickly ramify, 

 sending off their branches at nearly right angles, and are less flexuous 

 in their course than in the Petalodus ; they dilate into angular cells 

 at many parts of the mid-space between the medullary canals. 



In the genus Ceratodus the dental system is represented by a 

 single large plate on each side of both upper and lower jaws, (PI. 22, 

 fig. 2.) The detrition to which these plates were subject was re-- 

 paired by a continued development of the posterior part of the dental 

 plate in the manner which will be explained in the account of the 

 teeth of the Chimcera. 



The large and singularly sculptured dental plates, to which 

 M. Agassiz has given the name of Ctenodus, are likewise conjec- 

 tured not to have exceeded four in number in the mouth of the 

 extinct species indicated by these remains. The texture of the 

 tooth, like that of others of the Psammodus kind, presents a coarse 

 osseous structure at the base, supporting a dense osseous or enamel- 

 like layer ; the surface of the crown is minutely punctate. The 

 crown is traversed by twelve nearly parallel ridges, each of which is 

 notched or divided into a series of obtuse cones, which gradually 

 increase in size towards the outer border of the tooth. 



Such are the principal modifications of form and structure pre- 

 sented by the teeth of those extinct fishes, of which the Cestracion 

 of the Australian seas is the nearest living analogue. 



Were this genus to become extinct, all that would remain of it 

 in a few years would be its teeth and dorsal spines, the only hard 

 and durable parts of its frame. Fortunately we still possess the 

 evidence of the general form and organization with which they are 

 associated. But had even the teeth alone of the Cestracion remained, 

 a microscopic investigation of their intimate structure must have 

 led to an insight into the close affinities subsisting between the 

 extinct animal and higher cartilaginous fishes. For it is peculiar to 



