from the Shales of the Northumberland Coal-field. 361 



a low-pitched roof; and at last it entirely disappears, and is 

 replaced, as it were, bj a somewhat obtuse point of dentine. 



In the finest specimens, the whole tooth below the enamel- 

 cap is coated with a distinct film of enamel, which is perfectly 

 colourless ; in others traces of it are observed only here and there; 

 but in by far the greater number it is entirely wanting : when 

 this is the case, the surface of the tooth is frequently observed 

 to be roughened, as if by erosion. And it may be here stated 

 that it is not merely the enamel that is eroded, but it frequently 

 occurs that in the teeth of Palceoniscus, as Avell as in the teeth 

 of other small fishes, the dentine itself is worn away to such 

 an extent that very little of it is left to protect the pulp-cavity. 

 It is, therefore, not unlikely that all the teeth of Palceonisciis 

 were originally coated with enamel ; or it may be that in some 

 species tliere is an external coating of enamel, and in others it 

 is wanting. When the tooth is perfect, its walls are thick in 

 proportion to the calibre of the pulp-cavity ; the calcigerous 

 tubes are very fine and numerous. 



Note. — After the above description of the tooth oi Paloio- 

 niscus, it is scarcely necessary to say that there is no character 

 by which it can be distinguished from that of the so-called 

 genus Ganacrodus of Professor Owen (pi. 6) : the teeth of the 

 latter and former agree in size, form, and structure. We have 

 found the enamel-tip to exist in P. comtus and other species 

 from the marl-slate as well as in the species from our Coal- 

 measures. This we have proved in the most satisfactory 

 manner, not by taking the teeth at random as they are scat- 

 tered through the matrix, but by taking the jaws from the 

 heads of well-authenticated Palceonisci, and examining the 

 teeth both externally and in section. After having done this 

 in a great number of specimens, we are enabled to state that 

 the small enamel-tipped teeth found detached in the Cram- 

 lington and Newsham shales are exactly the same as those 

 attached to the jaws. They are of the same size and form, 

 with the same bright tip of enamel and finely fretted walls ; 

 and in section there is no difference whatever ; the general 

 form, the enamel-cap, the pulp-cavity, and dentine are all 

 precisely the same ; and all precisely agree with the tooth of 

 the so-called Oanacrodus. It is therefore hard to under- 

 stand what is meant by the use of such terms as " the 

 villiform teeth oi Amblt/jJterus and Palceontsacs,^^ "the vague 

 and ill-defined characters of those en hrosse of Palceoniscus 

 and Amhlypterusy Such expressions may indeed mislead, 

 as they or similar words appear to have misled their author ; 



Ann. & Mag. N. Hist. Ser.4, Voll. 26 



