78 



Guide to Trilobita. 



Table-case mososoma), which vary in numljcr from two to as many as twonty- 

 No. 17. nine, were movahly jointed togcthei- in the hving animal. Each 

 consists of a vaulted dorsal area (the tergum), and a Hat mem- 

 l)ranous ventral area (the sternum), and, on each side, a laminate 

 expansion overlapping the greater part or the whole of the legs. 

 Tlie 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 



Fig. 53. 



Examples of Trilobitcs. A — Calijmcnc bliimcnbacJiii (Uppci 

 Silurian). B — Ogygia bucliii (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 

 pi-obably 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 thev were able to double 



