326 CRETACEOUS ECHINODERMATA. 
Radiate animals of the orders Asteriadz and Ophiuride are seldom found in 
a fossil state, and when so found are rarely perfect. All star-fishes are fragile 
animals, and difficult of preservation ; hence the scarcity of their organic re- 
mains in ancient formations is not to be wondered at. ‘They appear to have 
commenced their existence at a very early period, for more than one species have 
been discovered in the Silurian rocks. They are known also as oolitic fossiis ; 
it is however in rocks of the cretaceous age that their fossil remains are found 
in greatest number and beauty. 
Living star-fishes are found in various depths of water, and on all varieties of 
sea-bottom : some live among rocks ; such are usually so formed as to adapt them 
conveniently to their habitat, have long and round arms and well-knit skeletons. 
Others living upon mud are more or less stellate and flattened. Pentagonal 
species live in all localities, and those with large marginal plates are usually 
from the deep sea in our regions, though in tropical countries they are found on 
the coral sand near the shore and in shallow water. 
The skeleton of the star-fish is a very complex and beautiful organism. It is 
composed of an immense number of little bones (ossicula), which are so articu- 
lated with each other, as to form a framework of great perfection and elegance. 
The ossicula vary in form in the several parts of the skeleton. The careful de- 
termination of their forms is of great consequence to the paleontologist, as no 
other parts of the animal are preserved in the fossil state. Fortunately the shape 
of an ossicle is as truly indicative of genus and species among the star-fishes, as 
that of a bone among the vertebrata; but hitherto little has been done in the 
study of the skeleton of the Asteriadz, and fossil species have mostly been defined 
on purely empirical characters. 
Every star-fish presents an upper and an under surface or disc, which in certain 
tribes are margined by two or more rows of large plates (marginal plates or 
ossicles). The centre of the disc is of greater or less extent according as the 
arms which radiate from it are longer or shorter. The surface is covered with 
a membranous and porous skin supported by small ossicula, various in form 
and more or less closely placed together. These extend over the surface of the 
arms also. On one side of the centre is a little plate different from all the others 
and marked by radiating grooves. This is the madreporiform tubercle ; not far 
from it, in most star-fishes, is a small pore regarded as an anal aperture. The 
surface of the plates of the disc and arms often bears spines, granules or tubercles ; 
also the curious pincer-like bodies called pedicellarie. At the extremities of the 
