ITS STRUCTURE, BULK, AND ASPECT. 83 



{)lates, identical in their character with those which covered 

 the head externally, and, like them too, thickly fretted with 

 tubercles, which, in the older and larger individuals, assumed 

 the normal star-like character, and in both young and old 

 manifested a tendency, where they approached the true teeth, 

 to assume, as in fig. 36, tooth-like forms. Fig. 37 represents 



Ficr. 37. 



PALATAL PLATE OF ASTEBOLEPIS. 



(One-fourth nat. size, linear.) 



the palatal plate of Asterolepis^ roughened with tubercles 

 somewhat larger than those of the external plates, but as 

 much of the same type as these, as the palatal shagreen of 

 Cestracion or Squalina is of the same type as that which 

 covers its back and sides. This tendency of dermal tuber- 

 cles to assume in some of the ancient ganoids the form of 

 teeth, and of teeth to assume in some of the existing placoids 

 the appearance of dermal shagreen, throws light on a class 

 of what would be otherwise very puzzling peculiarities in the 

 dental structure exhibited by not a few of the Old Red Sand- 

 stone ichthyolites. The teeth of the genera Coccosteus, As- 

 lerolepis, Diplopterus, and Cheirolejfyis, and at least the ichthyic 

 teeth of Asterolepisj Holoptychius^ and Glyptolepis^ seem to be 



