54 CESTRACIONTS. 



of conversion, not of excretion. As the teeth advance into use, the 

 organized pulp of each medullary canal becomes consolidated by the 

 formation of concentric osseous layers at its circumference, and by an 

 irregular calcareous deposition in its centre. The new teeth are 

 carried forward and outward by the same rotatory movement of the 

 membranes supporting them, as in the ordinary Squaloids. 



19. Acrodus. — Among the extinct Plagiostomes recognizable by 

 teeth or spines, the genus Acrodus, according to M. Agassiz, presents 

 the closest affinity with the existing Cestracion. The teeth of the 

 Acrodua, (PI. 14, fig. 1,) are of an oblong form with a convex or 

 slightly conical crown, supported on a coarse osseous base, in the 

 form of a parallelogram, obliquely truncated at its inner side. The crown 

 swells out beyond the base, and is expanded in the middle, and con- 

 tracted, generally unequally, towards the extremities ; it is traversed 

 by fine ridges, which diverge from one which runs along the middle of 

 the long diameter of the tooth. Portions of the jaw of the Acrodus have 

 been discovered which show that these teeth were arranged, as in 

 Cestracion in oblique rows, with, at least, seven teeth in a row. Those 

 at the anterior part of the jaws were short, with the middle of the 

 crown raised into an obtuse cone ; the posterior teeth were more 

 elongated and depressed. 



Professor Agassiz has remarked that ancient writers on Natural 

 History have mistaken the teeth of the Acrodus for fossil insects or 

 worms : by the quarry men of this country they are commonly called 

 petrified leeches : by some naturalists, they have been regarded as 

 auditory ossicles or otolithes of fishes ; but, independently of more 

 obvious evidence, a single inspection of a transparent slice of one of 

 these fossils in an adequate magnifying power, would suffice to de- 

 monstrate their real nature, (PL 14 and 15). 



The teeth of the Acrodus nobilis are composed of two substances, 

 viz : a thin external almost colourless layer, which represents the 

 enamel, and an amber-coloured coarser dentine composing the body 

 of the tooth, and continuous with and passing into its coarse 

 cellular bony basis and support. Microscopic sections of this 

 tooth aff'ord the most beautiful appearances, and, perhaps, the most 

 instructive illustration of the relation of dentine to bone. The body 



