ARTHEOPODA— TEILOBITES. 



83 



head-shield are the most closely united, and are of the same Gallery 

 number in all trilobites. Those of the thorax are flexibly Vlir. 

 joined, and in other genera vary from two to twenty-nine. 

 The pygidial segments, which also vary much in number, 

 are fused into a single shield, from which all traces of the 

 original segmentation has sometimes disappeared. The axial 

 portion of the head-shield, called the glabella, is separated 

 by the dorsal or axal furrows from the pleural portions or 

 cheeks, and is divided by side-furrows into lobes representing 

 the five primitive segments. The part of each cheek next 

 the glabella is firmly attached to it or " fixed." From this 

 the rest of the cheek is separated by the " facial suture," and 

 is " free." The free cheeks, which in many genera are 

 separate, here meet and fuse in front of the glabella. They 

 bear the compound eyes, which stand up like semi-circular 

 towers bearing facets on their outer walls. In many trilo- 

 bites the eyes are not so conspicuous, and in some they seem 

 to have been altogether absent. The hinder corners of the 

 head-shield, called the genal angles, are here produced into 

 spines. These are part of the fixed cheeks ; but in some 

 trilobites, where the free cheeks take a larger share in the 

 head-shield, they are part of those cheeks. Each segment of 

 the thorax consists of an axis and two pleura. The axis 

 stretches forward beneath the axis of the segment in front, 

 forming an articular surface. Each pleuron has a groove 

 running obliquely from front to back and connected with 

 the occasional overlap of the pleuroii in front as the animal 

 moved. Not all trilobites have the pleura grooved, but some 

 have them also bevelled at the front corners, forming facets 

 on which the hinder angles of the pleura in front could 

 play ; for these trilobites could roll up like a wood-louse. 

 The pygidium is jointed on to the last thoracic segment ; 

 its shield has a smooth border^ produced into a tail- 

 spine. 



The under side of the trilobite, with its appendages, has Table-case 

 been made known chiefly by the labours of C. D. Walcott ^^ll-case 

 and C. E. Beecher. A restoration of Triarthrus, a genus of i4b. 

 the family Olenidae, is exhibited (Fig. 39). The edge of 

 the carapace is turned over on the under side and supports 

 a thin membrane, in which are the two openings of the 

 digestive tract : the mouth, beneath the glabella, and the 

 anus, beneath the pygidial axis. In front of the mouth is 

 the hypostoma or fore-lip, behind it is the metastoma or hind- 



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