28 
traverse itself being invisible. He had not satisfactorily 
made out the structure of the second substance ; it resembled 
the pith of plants, but it was less regularly cellular, and in 
some spines assumed a fibrous appearance. It is opaque 
under transmitted, and glistening white under incident 
light. In the following remarks he called this opaque, and 
the first described transparent substance. The general 
structure of the spines he had examined was also twofold ; 
in the simpler, as the Amphidotus cordatus, the centre por- 
tion is hollow ; in the more complicated it is composed of 
the opaque substance perforated along the length of the 
spine by vertical solid tubes of the transparent matter, with- 
out any definite arrangement. These appear to increase only 
in length ; hence a section at the apex of the spine shows 
in the centre a prolongation of the oldest portion, the 
thickening of the spine arising differently, as subsequently 
explained. The hollow centre of the Amphidotus cordatus is 
surrounded by a cellular fretwork of the transparent matter, 
while around this is a circle of solid ribs or pillars of the 
same, smooth on the exterior of the spine, but within beauti- 
fully hollowed out into what the heralds call an engrailed 
outline, the points of which connect it with the inner layer 
of cellular fretwork. This framework is occasionally want- 
ing, and the engrailed points are simply connected with 
each other by a straight inner line of transparent matter. In 
the more complicated forms he was not satisfied he had ascer- 
tained the real structure, but thought it to be as follows.— 
The tubes of transparent matter noticed about the central 
opaque substance, as they approached towards the circum- 
ference of the first season’s growth, gradually coalesce, and, 
