CRUSTACEA. 145 
large size, and are often called “ Molucca crabs” from their 
occurrence in the Moluccas. Both the eggs and the flesh are 
eaten by the Malays. 
Closely allied to the king-crabs is the extinct family of the 
Eurypterida, an example of which is figured above (fig. 67). 
This species is supposed to have attained a length of probably 
six feet, but other forms were very much smaller. 
OrDER TriILoBiTaA.— The Trilobites constitute another 
wholly extinct order of the Crustacea, and deserve a short 
notice from their great geological importance. They derive 
their name from the fact that the body exhibits a more or less 
conspicuous division into a central and two lateral lobes (fig. 
68, 1). The entire shell or crust is composed of an anterior 
semicircular shield, covering the head (fig. 68, 2), a series of 
Fig. 68.—Trilobita. 1. Azgelina Sedewickii; 2. Diagram of the cephalic 
shield of a Trilobite. (After Salter.) 
movable rings, constituting the thorax, and a tailpiece com- 
posed of amalgamated segments, and representing the ab- 
domen. On the under surface of the shell nothing had ever 
been discovered except the upper lip, but recently traces of 
limbs have been made out. The cephalic shield usually bears 
a pair of compound eyes (fig. 68, 2 a), but these are sometimes 
wanting. It is probable that most of the Trilobites possessed 
the power of rolling themselves up into a ball, much as our 
modern wood-lice. The Trilobites are only known as occur- 
ring in the older rocks of the earth’s crust, and they are 
chiefly characteristic of the period known to geologists as the 
“ Silurian.” 
K 
