CESTRACIONTS. 57 



coarse, osseous, wrinkled base, and an enamelled crown. The crown 

 is separated from the base by a slight constriction, above which it 

 swells out and then quickly diminishes to an apical ridge, which 

 traverses its long diameter, and presents a series of pointed cusps ; 

 of these, the middle one is the largest and highest ; the rest quickly 

 diminish in size as they recede from it. The sides of the crown are 

 traversed by well marked but fine ridges, which converge from below 

 towards the middle cusp, but are variously disposed in the different 

 species of Hybodus. 



The cusps of the exterior teeth are erect, those of the internal or 

 posterior ones are recumbent, but the transition from one position 

 to the other is gradual and progressive as in Squatina and the anterior 

 teeth of Cestracion. The development of these teeth corresponds 

 with that in the Cestracion, where the whole breadth of the pulp is 

 progressively ossified from above downwards, instead of from the 

 external surface of the whole crown inwards ; hence, the last formed 

 teeth are not hollow, as in the true sharks. 



The teeth of some species of Acrodus, as the Acrodus minimus 

 of Agassiz, resemble so closely those of a Hybodus in external form, 

 as to be liable to be mistaken for those of that genus. The micros- 

 copic structure of the teeth, in these closely allied genera, is very 

 similar in its general plan. In Hybodus, however, the calcigerous tubes 

 in the body of the tooth have a less wavy disposition than in Acrodus. 

 21 . Ptychodus. — The fossil teeth, on which M. Agassiz has established 

 the genus Ptychodus, have not as yet been discovered undisturbed in 

 their natural position in the jaws, so as to demonstrate the affinity 

 of the extinct fish to the recent Plagiostomes, with the same certainty 

 as in the case of the Hybodus and Acrodus ; nevertheless, their 

 number, their external form, and the absence of any other parts of 

 the skeleton in the localities where they are most abundant, alike 

 bespeak that they belonged to a cartilaginous fish, which M. Agassiz 

 conjectures to have been nearly allied to the Cestracion. The micro- 

 scopic texture of these interesting fossils, which 1 described at the 

 meeting of the British Association in 1838, afforded the demonstra- 

 tion finally required of their true nature. The entire tooth (PI. 17, 



