ITS STRUCTURE, BULK, AND ASPECT. 



8^ 



NAIL-LIKE BONE OF 

 HYOID PLATE. 



(One-half nat. size.) 



the plate in the opposite page, occurred, — 

 for its general form is different (tig. 41), — 

 there appear well-marked ligamentary im- 

 pressions, closely resembling that little 

 spongy pit in the head of the human thigh- 

 bone to which what is termed the round 

 ligament is attached. The entire hyoid- 

 plate, viewed on its outer side, resembles 

 in form the hyoid-bone, — or cartilage 

 rather, — of the spotted dog-fish {^Scyllium 

 steUare) ; but its area was at least a hun- 

 dred times mere extensive than in the 

 largest Scyllium, and, like all the dermal 

 plates of the Asterolepis, it was thickly 

 fretted by the characteristic tubercles. In 

 the ray, as in the sharks, the piece of thin 

 cartilage of which this plate seems the homologue is a flat, 

 semi-transparent disc ; and there is no part of the animal in 

 which the progress of those bony molecules which encrust 

 the internal framework may be more distinctly traced, as if 

 in the act of creeping over what they cover, in slim threads 

 or shooting points, and much resembling new ice creeping 

 in a frosty evening over the surface of a pool. 



The two angular terminations of the hyoidal plate, a, a, 

 fig. 40, were received, laterally and posteriorly, into angular 

 grooves in a massive bone of very peculiar shape (fig. 42), of 

 which the tubercled portion, a, a, seems to have swept for- 

 wards in the line of the lower jaw, forming the rounded edge 

 where the flat under part of the creature's head merged into 

 the lateral part of it, as the bottom of a portmanteau merges 

 into its sides ; while the delicately grooved projection, b, 

 struck across towards the point of the hyoidal naiL At the 

 termination of this transverse projection nearest the side of 

 the head, and where there runs towards it through the tu- 



