Trilobita. 
t / 
Class 2.— TRILOBITA. 
The members of this class are known only in the fossil state, Table-case 
and are characteristic of the strata of the Palaeozoic era. They No - 17 - 
are especially abundant in the Silurian and pre-Silurian rocks. 
On the whole, they seem to be most closely related to the 
Arachnida, and especially to the Xiphosura or King-crabs, but in 
certain features they resemble the Crustacea, and some authorities 
are of opinion that they are allied to that class. The somites of 
Fig. 52. 
Reconstruction of a Trilobite, Triarthrus becld. 
Natural size (after Beecher). 
the body are variable in number, each, so far as is known, being 
provided with a pair of appendages which, with the exception of 
the pre-oral pair, are substantially similar in structure and function. 
The dorsal plates of the five somites composing the anterior 
region of the animal (the “ head ” or prosoma) are fused to form 
a carapace or “ cephalic shield ” ; its median area is vaulted, and 
each of the lateral areas is expanded, laminate, and divided by a 
groove into an inner and an outer portion ; upon the latter a large 
compound eye is present. 
The somites of the middle portion of the body (thorax or 
