CRUSTACEA. 97 



Trilobites. 



These extinct Crustaceans were the earliest members of the class, 

 meeting us in the first formations in which we have any abundant forms 

 of marine life. They do not belong to any modern group. They 

 resemble both the Entomostracans and Tetradecapods, but have an 

 independent position. They have been likened to the larvae of 

 the Horse-Shoe Crab. The structure of the tergal part only of their 

 body is well known. The longitudinal division into three lobes is 

 not always distinct ; but the head, thorax and abdomen (or tail) are 

 generally well-defined. The head is covered by a crescent-shaped shield 

 or buckler : the thorax consists of from two to twenty segments ; and the 

 pygidium, or abdominal shield, is composed of a number of joints more or 

 less anchylosed. The buckler is divided by a longitudinal depression 

 into cheeks, or lateral areas and the glabella, or middle area. A suture 

 running from the anterior and posterior sides of the eye outward is called 

 the facial suture ; a prominent accessory piece on the under surface of 

 the head, covering the mouth, is the hypostoma. The union of the head 

 crust by sutures is a remarkable peculiarity of Trilobites, — not. being 

 traceable in any other Crustaceans, and only found in true Insects of 

 the present world. The genera of Trilobites are distinguished mainly 

 by the form and markings of the head and tail, and by the eyes. Varia- 

 tions in the length of the cephalic and caudal spines and in the promi- 

 nence of the head-lobes, have been considered indications of difference 

 of sex. No antennae have been detected, nor any feet (save in one speci- 

 men of Oalymene) : the latter are supposed to have been rudimentary or 

 membranous. They probably lived gregariously in the shallow waters 

 of bays and coasts, swimming on their backs j and from the form of the 

 labrum of the mouth, it is inferred that they were carnivorous. The 

 embryos are simple discs. Barrande has made out the probable eggs. 

 More than 400 species of Trilobites are known, grouped in about 50 

 genera. Of these, 46 are Silurian, 22 Devonian, and 4 Carboniferous. 

 According to Bronn, 13 genera are peculiarly Lower Silurian, 3 Upper 

 Silurian, 1 Devonian, and 3 Lower Carboniferous. The spinous feature 

 reached its maximum in the Devonian. Trilobites, incapable of rolling 

 themselves up, are of more ancient geological date than those which 

 can : they are chiefly found in the Lower Silurian. Those having the 

 power of rolling themselves up, have larger and more projecting eyes, a 

 tougher horny membrane, and a caudal shield resembling the buckler. 

 Trilobites not granulated occur only in the most ancient and the newest 

 Pakeozoic strata ; while the granulated species are found in the middle 

 beds. 



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