BOOM II.] NATURAL HISTORY. 63 
which form the support of the body, and which generally 
agree with the external form of the body. 
The family of Astropectenidce (Cases IS and 14,) have 
the back of the rays thin and netted, with numerous tuber¬ 
cles at the junction of each of the little bones, which are 
covered at their tips with numerous small moveable spines. 
They are the most beautiful of the order. 
The family of Pentaceridce (Cases 15 to 17,) have the 
skeleton formed of large roundish tubercles, the skin be¬ 
tween which is pierced with small holes ; some of these 
have a convex back and triangular arms, as the Pentace - 
ros; others are flat above and beneath, with five broad, 
short rays edged with larger pieces, as Goniaster ; many 
of these have their back studded with conical tubercles, 
which enlarge in size as the bone on which they are placed 
increases ; these tubercles sometimes fall off as the animal 
increases in size, and are easily separated, leaving a flat 
scar when the specimen is dead. Some of them, as the 
Dactylosaster , Case 17, have elongated cylindrical arms; 
The last family, or the Asterinidce , have the skeleton 
formed of compressed pieces, placed one over the other 
like the tiles on a house ; they always have a thin mar¬ 
gin. They are generally flat and pentangular, but some 
have a convex back, and a few are many rayed. (Case 18.) 
The Lizard-tailed Starfishes, Ophiurida, (Cases 
19 to 24,) have a suborbicular depressed body with five 
cylindrical, jointed, very flexible arms, which are some¬ 
times repeatedly divided into branches ; the arms are 
furnished with a series of small pores, along each side of 
their under or oral surface, and they have a lunate hole on 
each side of the base of their arms. They have no dorsal 
warts. 
The family Ophiuridce have an orbicular body covered 
with spines or scales, and the arms, which are always 
simple, are furnished with cross series of spines on the sides. 
In some, ( OpJiiosoma , Cases 19, 20,) the spines of the arms 
are long and expanded, in others, as Ophiura , they are short 
and appressed. Most of the species have five rays, there is 
one in the collection (Case 20) with only four, and some 
have been described as having eight, but they are only 
monstrosities. 
The family of Gorgon heads, ( Euryalidce ,) on the contrary. 
