SQ THE ASTEROLEPIS, 



described as one of the most characteristic bones of the A»- 

 terolepis, belonged. In the Osteolepis, the space correspond- 



Fig. 40. 



a a 



HYOED PLATE. 



(One-ninth nat. size, linear.) 



ing to that occupied by this hyoid plate was filled, as shown 

 in fig. 1 4, by five plates of not inelegant form ; and the di- 

 visions of the arch resembled those of a small Gothic window, 

 in which the single central muUion parts into two branches 

 atop. In the Holoptychius and Glyptolepis there were but 

 two plates j for the central muUion, i. e. line of division, did 

 not branch atop ; and in the Asterolepis, where there was no 

 line of division, the strong nail-like bone occupied the place 

 of the central mull ion. The hyoidal armature of the latter 

 fish was strongest in the line in which the others were weakest. 

 Each of the five hyoid plates of the Osteolepis, or of the two 

 plates of the Glyptolepis or Holoptychius, had its own centre 

 of ossification ; and in the single plate of Asterolepis, the 

 centre of ossification, as shown by the radiations of the fibre, 

 was the wai^-head. This head, placed in immediate contact 

 with the strong boxes of bone which composed the under 

 jaw, just where their central joining occurred, seems to have 

 lent them a considerable degree of support, which at such a 

 juncture may have been not unnecessary. In some of the 

 nail-heads, belonging, it is probable, to a different species of 

 Asterolepis from that in which the nail figured in page 7, and 



