304 THE MEDALS OF CREATION, Cuap. VIII. 
position of the soft parts, we can understand how layer upon — 
layer of ossicula of Asteriadee may have been formed at the 
bottom of the cretaceous seas ; as we find them in the quarries 
near Arundel, Worthing, &. Whoever has witnessed the 
hauling up of the dredge off our coasts, and seen the mass 
of living Star-fishes which it brings up, as if the sea-bottom 
were a living bank of these Radiata, will not be surprised 
at the vast quantities of their fossil remains. This profu- 
sion of the living animals of this family, serves also to 
account for the enormous amount of those kindred but ex- — 
tinct forms, whose relics were the subject of investigation in 
the former part of the present chapter. 
The ossicula vary in shape in different parts of the skele- 
ton, and Prof. E. Forbes affirms, that the careful determina- 
tion of their characters is of great importance, since they are 
the only parts of the animals likely to be preserved, and the 
shape of an ossicle is as truly indicative of a genus or species, 
as is that of a bone among the vertebrata. There is one 
ossicle situated on the side of the centre of the disk, which 
is worthy of remark, because it often occurs in the chalk 
mingled with the debris of the rest of the skeleton, and 
should be preserved by the collector. It differs from all the 
other bones in being marked with radiating grooves, and is 
called the madreporiform tubercle; it appears to be the 
analogue of the stem of the Crinoidez, in other words, a 
rudimental condition of an organ, which is fully developed 
in that order of radiata. 
OpniuRA. Lign. 97, fig. 1.—Several species of this genus, 
which is distinguished by the long, slender, serpent-like arms, 
and the circular disk covered with plates and spines, have 
been found in a fossil state: one species was discovered in 
the Lower Silurian deposits by Prof. Sedgwick, and other 
forms have been obtained from all the succeeding formations. 
The Lias near Lyme Regis and Charmouth has yielded many 
