AETHEOPODA — TEILOBITES. 
83 
liead-sliiekl are the most closely united, and are of the same 
number in all trilobites. Those of the thorax are Hexihly 
joined, and in other genera vary from two to twenty-nine. 
The pygidial segments, which also vary much in number, 
ai'e 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 tlie “ 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 pleuron 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 spine. 
The under side of the trilobite, with its appendages, has 
been made known chiefiy by the labours of C. D. Walcott 
and C. E. Beecher. A restoration of Triarthrus, a genus of 
the family Olenidae, is exhibited (Fig. 39). The edge of 
the caraj)ace 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- 
lip. Specimens of other species showing these structures 
are exhibited. Every body-segment, except that in which 
G 2 
Gallery 
VIII. 
Table-case 
25 . 
