GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



233 



served for comparison, measures from tip to tip of two op- 

 posed rays four inches, and from the disk to the tip one and 

 three-fifths inches. It is very probable that the tB.. antiqua 

 was furnished in its living state with movable spines, like 

 the generality of those belonging to this division, but these 

 spines have disappeared in our specimen, it being imbed- 

 ded, in fact partly incorporated, in a solid granular lime- 

 stone, and has only come to light by the wearing of the 

 rock, by which are also obliterated most of its specific 

 characters. Not only the spines are worn oif, but it 

 seems that also the epidermis has disappeared. We see 

 from the figure, (see plate 10, fig. 9), that the margin 

 which encloses the groove is composed of two difi^erent 

 parts, the outer part being composed of articulations or 

 pieces of a parallelopipedon shape, except the first, 

 placed nearest to the disk, which has the form of an equi- 

 lateral triangle, one of the sides placed against the disk. 

 The internal part of the margin is narrower than the 

 external one, and is composed of a double row of trian- 

 gular points, placed in such manner, that the groove 

 seems surrounded by white triangular points placed close 

 to one another, while the intervals form a row of black 

 triangles. Perhaps this appearance is partly owing to 

 the effect of fossilification — or must these be considered 

 as the internal structure of the animal ? In the latter 

 case, it diff*ers widely from those that are living on our 

 shores. I have, in order to see whether I could pro- 

 duce a similar appearance, cut ui^ ai dry »Bsterias^spinosa. 

 Having removed the spines, I have successively worn 

 down the whole animal to the cavity in the rays, and 

 I was not able to produce any thing similar to the fossil 

 species. 



I am not acquainted with the internal structure of the 

 articulated species mentioned by naturalists. Perhaps 

 its structure may be similar to ours. As the whole of its 

 externa] surface has disappeared, it is impossible to de- 

 I.— 2 E 



