Trilobila. 



Class 2. TRILOBITA. 



The members of this class are known only in tlie fossil state, Table-case 

 and are characteristic of the strata of the Palaeozoic era. Tliey ^^<^- ^'^• 

 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 authoi'ities 

 are of opinion that they are allied to that class. The somites of 



Fig. 52. 



Reconstruction of a Tiilobite, TriarthruH bccki. 

 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 l)ody (thorax or 



