REMAINS OF REPTILES AND FISHES. 99 



thick cap of enamel, and is usually quite smooth and highly pol- 

 ished. Below the cap, in all the species examined, the crown 

 has a subdued lustre, and is fretted in a very beautiful manner 

 with numerous minute, short, close-set, longitudinal depressions, 

 which, being arranged lengthwise, have occasionally a lateral in- 

 clination : hence the peculiar fretted appearance of the surface. 



On making a longitudinal section (Plate III., fig. 2), the pulp- 

 cavity is seen to conform to the shape of the crown ; the cavity 

 is wide below and narrow above, tapering gradually towards the 

 apex, and terminating just within the extremity of the dentine. 

 The tip of enamel fits on to the top of the dentine like a ferrule, 

 and is in the form of an inverted V, with the angle filled up for 

 some distance, and the stout limbs turned out a little below, and 

 mortised, as it were, into the dentine. The enamel-cap varies 

 a little in form in the different species ; but it varies still more 

 in accordance with the plane of the section. When the section 

 is made directly through the centre, the solid apical portion of 

 the enamel is seen to be much produced, and very sharp. By 

 making the section a little eccentric, the solid tip is reduced 

 in length and sharpness ; and by carrying the process a little 

 further, the enamel- cap becomes a mere thin covering, like a 

 transverse section of a low-pitched roof; and at last it entirely 

 disappears, and is replaced, as it were, by 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 ob- 

 served 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 fre- 

 quently occurs that in the teeth of Palmoniscus, as well 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 Palcc- 

 oniscus were originally coated with enamel; or it may bo that 

 in some species there is an external coating of enamel, and in 



