2SO 



NORTH AMERICAN INDEX FOSSILS. 



Phylum VII. ARTHROPODA. 



Class Crustacea Lamarck. 



Subclass Trilobita Burmeister. 



Extinct marine Crustacea wholly confined to the Palaeozoic 



rocks. Body covered with a shield {dorsal shield or carapace) 



longitudinally divided into three parts. 



The anterior portion comprises the head-shield or cephalon, 

 which is usually semicircular, with a straight posterior border. 

 The central of the three lobes of the cephalon is the glabella which 



Fig. 1542. Diagram of a trilobite to show the parts. (After Clarke.) C\ cepha- 

 lon ; g, glabella; /", frontal lobe; I, 2, 3, first, second and third lateral lobes; i'', 

 2^, y, first, second and third lateral furrows ; x, fixed cheeks ; fc, free cheeks ; s, 

 facial suture; e, eye; n, palpebral lobe; og-, occipital groove (neck furrow) bound- 

 ing the occipital ring (neck ring); d, border. 



T, thorax : a, axis ; /, pleurae ; s, first thoracic segment ; ag, articulating groove ; 

 pg-, pleural groove. 



/*, pygidium : an, annulation ; pr, pleural rib ; cs, caudal spine or telson. 



is the most prominent part of the cephalon. It is of varying out- 

 line, bounded laterally by the dorsal furrows, and more or less 

 divided by transverse furrows or pairs of furrows. The last 



