OPHIURIDZ. 271 
This disc is from six-eighths to seven-eighths of an inch in size, the 
upper part covered with unequal plates, in shape like tiles ; the arms 
are four times the length of the diameter of the disc, very slender and 
tapering. The Ophiuroid to which Lamarck gave the name of 
Fig. 109.—Ophiocoma Riisei (Lutken), natural size. 
Ophiura fragilis has now its place in the genus Op/zothrix, the specific 
name indicating a peculiarity of structure in all these small creatures 
derived ftom their fragile formation. In short, these beings have so 
little consistency, that many of them crumble as'it were under the 
touch, and become reduced to pulp under the slightest pressure. 
In Fig. 109 we give the representation of an Ophiuroid of the natural 
