THE DERMOSKELETOX 



19 



geons, the dermal bones are coated externally with a 

 much harder material, resembling eaamel, and such fishes 

 have accordingly been termed "ganoid," from the Greek 

 word " ganos," signifying brightness. The ganoid plates 

 in those extinct fishes are "usually more close-set, over- 

 lapping each other, and being fastened together like tiles, 

 by a peg of one entering a socket in the next, and recip- 

 rocally. Only two genera of fishes are now known to 

 exhibit this beautiful arrangement of the dermal bones, 

 viz. the ]^oly]r)terus of the Nile, and the lepidosteus of the 

 Ohio, and other great rivers of North America. 



In the armadillo, the dermal bones, Fig. 2, o5, are small, 



PORTIONS OF DERMO AND XEURO SKELETONS — ARMADILLO {DciST/JiUS trichictus). 



polygonal, usually five or six-sided, smooth on their inner 

 surface, which rests on the soft subcutaneous layer of cel- 

 lular tissue, variously sculptured on the outer and ex- 

 posed side, but with a pattern constant in, and character- 

 istic of, each species. They are united together at their 

 thick margins by rough or " sutural" surfaces, and resem- 

 ble a tessellated pavement. The trunk is protected by a 

 large buckler of this bony armor; the head is defended 



