54: 
diameter of the upper one, and has its surface roughened by 
small tuberculations, but for a very short distance below the 
upper rod resembles it in character. 
Ambulacral tubes.—The ambulacra bearing their tubes are 
prolonged on the peristomal membrane to the opening in its 
centre, through which the teeth of the animal project; these 
ambulacral tubes are well developed here and on the under 
surface of the shell generally, but on the upper or dorsal 
region are reduced to a rudimentary state, and probably only 
assist in the process of respiration. _ The free sucker-like ex- 
tremity of the tube is kept distended by a system of plates 
similar to those found in other Echinoidea, but as its structure 
can be demonstrated more readily in some of the larger forms, 
I shall for the present reserve its description. The general 
body of the tube is strengthened and supported by numerous 
spicula (fig. 6), the form of which is peculiar to the Cidaride ; 
they are found as cylindrical, rough fibres, curved to adapt 
them to the form of the tubes, to which they give a ringed 
appearance when 7 situ; they are roughened by numerous 
short spinules, which are most abundant on their convex or 
outer border; the length of each spiculum is about equal to a 
quarter the circumference of the tube. 
The various parts whose minute structure has been now 
described are confined to, or are appendages of, the external 
surface of the corona or shell; the characters they furnish are 
constant to, and only found in members of the family 
Cidaride. They vary only to a slight extent in the different 
genera and species. 
The soft viscera, &c., found in the interior of the Echinoidea 
are in Cidaris supported and rendered firm and rigid by the 
formation of numerous spicula in the delicate membrane which 
lines the interior of the shell, and is reflected over them. 
Their forms present features by which genera and species 
seem clearly to be indicated. This condition is, with the ex- 
ception of one genus, constant to the family, but must not be 
considered as peculiar to it, as some members of the 
Echinometride show a similar structure. ‘The spicula of the 
ovary are, I believe, the most important as furnishing a 
character which, though modified considerably in the different 
genera, is, as far as I have yet seen, essentially the same in 
each genus. ‘The spicula of the other viscera are more varied 
in their forms, and would probably be of assistance in de- 
termining species. 
Ovary.—In dried specimens of Cidaris the branched 
tubes composing the ovary do not retain their individuality, 
but the ovary appears as a flattened cake, adhering to the 
