133 
NORTH ZOOL. GAL.] NATURAL HISTORY. 
teeth. (See Case 3.) These shells are generally covered 
with large spines. The Diadems ( Cidaridce ), Case 1, have 
the tubercles on which the spines are affixed pierced with 
a central pit, and the Echinidce (Cases 2 to 6) have, on 
the contrary, simple rounded tubercles. The spines of 
most of the species of this family are equal-sized, but in 
some, especially such as are of an oblong shape as 
Echinometra lucunter , the spines are large and club- 
shaped, and in others, as E. atratn , they are very short 
and truncated, forming, in the mass, a smooth surface 
somewhat resembling a tessellated pavement. These 
animals are much sought after as food during the latter 
part of the summer, at wffiich time they are almost filled 
with eggs. 
The other Echinida have the vent placed on the side 
of the body. 
The family of Scutellidce (Cases 7, 8) have the rounded 
mouth in or near the centre of the under side. Their 
shell is covered with very minute equal-sized spines, and 
the pores form arched series like the petals of a flower, 
on the middle of the back. Their jaws are complicated, 
triangular, inclosed, and formed for crushing the food. 
The shells of these animals are strengthened by columns 
of calcareous matter which are deposited in the cavi¬ 
ties between the folds of the internal organs. Some 
species are convex, as Echinantkus , but they are mostly 
very flat and depressed, as the Scutella. Many of the 
latter are pierced with holes in the disk, or are lobed on 
the edge. 
The family of Galerit'idce (Case 9) resemble the former 
family in the position of the mouth and vent, and in being 
covered with very small spines, but the lines of pores ex¬ 
tend from the middle of the back to the mouth ; they are 
sometimes interrupted at the margin, but then they are to 
be seen again forming five distinct petal-like impressions 
on the under side near the edge of the mouth. The cavity 
of the shell is simple, and their jaws are small or only 
rudimentary. Only a few kinds are now found in a recent 
state, but the extinct species are very numerous, forming 
several very distinct genera. The vent in some of these 
is above the hinder margin, but it is never in the middle 
of the back. 
