78 
Guide to T rilobita. 
Table-case mesosoma), which vary in number from two to as many as twenty- 
N°. 17. nine, were inovably jointed together in the living animal. Each 
consists of a vaulted dorsal area (the tergurn), and a flat mem- 
branous ventral area (the sternum), and, on each side, a laminate 
expansion overlapping the greater part or the whole of the legs. 
The convexity of the terga and of the upper surface of the lateral 
laminae gives to the body a three-lobed appearance, from which 
the name Trilobita is derived. The dorsal and lateral plates of 
the somites of the posterior region of the body (pygidium or 
metasoma) are immovably united, although generally defined by 
transverse grooves. 
The appendages of the first pair, where known, are each in the 
form of a single long, branched, antenniform limb. Those of the 
Examples of Trilobites. A — Calymene blumenbachii (Upper 
Silurian). B—Ogygia buchii (Ordovician). 
remaining pairs consist of two branches rising from a common 
basal segment. The external branch is slender, many-jointed, and 
furnished with a series of slender branchial filaments ; the internal 
branch, constituting the locomotor portion of the limb, consists 
of six or, including the basal segment, seven segments. The post- 
oral appendages of the prosoma resemble those of the rest of the 
body, except that the inner extremities of the basal segments are 
toothed to act as jaws. 
The Trilobites are an extinct group of marine Arthropoda which 
probably resembled the existing King-crabs in habits, and crept 
about the bottom of the sea, feeding upon worms and other soft 
animal organisms, which were crushed between the basal segments 
of the anterior appendages. On account of the softness and 
membranous nature of the sternal region they were able to double 
